My mum looks uncharacteristically ­serious. “If ­anything happens to me, call this number.” “What – like you get assassinated by a gang of international spies, or fake your own death to start a new life in Brazil?” I say, as I take the card. It’s for a local solicitor, and for some reason it’s canary yellow.

“Don’t worry, I plan to stick around for many years to come. Just, you know. Take it anyway. Just in case.”

“Is this about … him?”

“I picked up a box of doughnuts at the station.” She deftly changes the subject. “I know you said not to, but you’re so thin …”

It’s February 2018, sleeting rain driving on to the window of my one-bedroom flat in south London. I recently moved in, and my mum, intensely practical in some ways, has come over to help me install a new sink trap. We’re on OK terms, but we see each other maybe twice a year, and it’s always awkward. There’s a lot of water under the bridge.

I’ve told my mum a thousand times that I don’t have a sweet tooth, that I’d prefer crisps or cheese, but she always has a hard time remembering that I’m not a mini version of her. Hence the endless parade of cookies and doughnuts. I put the business card down and immediately forget all about it.

“I’ve broken up with him,” she blurts out an hour or so later, her face wobbling. I keep my face blank. I’ve heard this so many times before. She tries to tell me about her boyfriend’s latest bad behaviour and I shut down. How dare she use me as some kind of relationship therapist, for this man, for this relationship? “No, I mean it this time. I called a domestic violence charity, and I started seeing a new therapist who specialises in domestic abuse. They’re helping me evict him.”

“OK,” I say. Just that: OK.

This man entered our lives when I was 14 and my mum was in her late-40s and smarting from a brutal divorce. He’d roared up on his motorbike, a thirtysomething with muscles, a permanent can of beer in his hand and a Marlon Brando attitude. He was the complete opposite of my nerdy mathematician father, who loved early modern music and couldn’t handle being in a room that contained more than about four people. This man was a drama school graduate and wannabe actor; he played his beloved On the Waterfront VHS tape over and over, talking about what a kinship he felt with the role, how he would be the next Brando. Then he’d ask my mum to pay for the takeaway, always “a bit short that month”. He couldn’t possibly get a day job: he had to keep his schedule free for auditions.

It’s tempting to paint him as the villain, and my family as the victims. But my family was damaged long before he entered the picture. I loved my mum and dad and they loved me and they did their best, but they weren’t always great parents. When I was 12 everything fell apart, all at once. I had a horse-riding accident and was bedridden for six months with what ultimately would be diagnosed as ME. During this time my parents’ marriage disintegrated. A divorce judge decided to “prioritise the wellbeing of the child” by ruling that I should be allowed to remain in the family home, meaning whoever got custody of me got the house. It was well meaning, but it put a price on my head. For 18 months my parents were trapped in the same house as they fought a bitter custody battle that I felt was more about who got the house than who genuinely wanted me.

A lot of awful things happened during those 18 months. The police were regularly called. My parents had pressure put on them to deregister me from school because ME wasn’t well understood then, and both of them had temporarily stopped working, so the three of us were stuck in this pressure cooker. They were both as bad as each other, and I gave as good as I got. (No villains, no victims.) I knew my dad couldn’t look after me, so I told the judge I wanted to live with my mum, and the judge told my dad to move out. I thought it would just be mum and me living in our beautiful girly house together, but she wanted to be free, she wanted to feel sexy and desirable for the first time in a long time. I was pretty hostile when she immediately turned round and moved a bloke in. I know that doesn’t make anything that happened afterwards my fault.

And the first time he shoved his way into my bedroom because I wasn’t strong enough to hold the door closed, the first time he hit me, the first time he smashed a chair over my head, the first time I called the police and the first time they didn’t believe me, she didn’t want to know. When other, worse things happened, I knew not to bother calling the police. It felt safer sometimes to take the tube into central London and walk around all night, or sleep in a park.

Years passed. I hadn’t been in school or received any form of education since just before I turned 13, so I had lost contact with all my friends, too. My relationship with my parents became strained and then snapped completely. Before too long the system stepped in and turned me into a problem to be parcelled off. I was put into B&Bs, into random box rooms in strange parts of the country, then, finally, at 17 dumped in a bedsit with the rent paid and left to get on with things. And as I started the slow process of putting my life back together and the world gradually opened up to me, my mum was falling into domestic abuse and her world was shrinking.

***

I don’t remember who first picked up the phone, after years of estrangement. My mum had to keep her relationship with me a secret, to avoid making him angry. I wasn’t allowed to call her. I had to text (I was in her phone under a fake name) and she’d call me back while she was walking her dog, because it was the only private time she had. I learned to not have any crises when it was raining.

She told me there’d been incidents of violence, but I think not many. Coercive control and emotional abuse are harder to define but can be equally damaging. Her boyfriend didn’t let her speak to me, because I was the monster who’d called the police on him. She wasn’t allowed to speak to her friends, because they were trying to poison her against him. Her therapist was against him, too, the man-hater. She was obviously far too fragile to go out to work; find a nice work-from-home job instead. And did she really need to go into town for lunch? Abuse can be subtle; I don’t know the precise dynamics of their relationship or why she didn’t leave. All I know is that my mum became a shadow of herself. She lost contact with her friends. She barely went out. She paid all the bills, bought a houseboat and thousands of pounds of woodworking equipment for a man who had never held down a job in his life, but was 15 years her junior and made her feel young and cool and desirable.

And throughout all this I was focused on myself, and on the mountain I was climbing. I got myself back in education. I sat exams as an adult then applied to university as a mature student, then postgraduate school. I somehow became a playwright and found out that I was good at it. I was so proud. I wanted to be the star of my own story for once and I didn’t want to keep being dragged down by my mother’s constant drama. I felt sorry for her but I mainly just wanted to shake her. If you’re choosing to wake up every day and cook dinner for the man who abused your child, I don’t think you get to centre yourself as the victim. I know that no one chooses to be in an abusive relationship, but I also can’t write a narrative where my mother is the victim, because real life isn’t that simple.

And then she died. So I guess she is the victim of this story because she’s dead, and if anyone’s the victim it’s sure as hell not me.

I want to tell you about my mum. I want to make her a person to you. She was insecure about being 5ft 1in and having a bump on her nose. She loved animals more than anyone. If she saw a hedgehog on the motorway she’d stop and leap out, racing across three lanes of traffic to save it. It took her five years to get pregnant with me: she was about to start IVF with the fertility pioneer Robert Winston when she put a note in the Western Wall in Jerusalem asking God for a baby, and she got pregnant that same week. She told me she was fine with an infant, but the minute I learned to talk it was game over, because I had such a strong and forceful personality; she knew that she, as a “fundamentally very weak person”, could never stand up to me. I used to think that was a compliment. When I was 13 and pretty much bedridden, I’d sometimes be in so much pain I couldn’t sleep or think or do anything. She’d pack me into the car late at night and drive me around London. She longed to be an actress or a writer more than anything. She did a degree in English literature when she was 40, having left school at 16. Her rottweiler/black lab cross, Charlie, was the joy of her older life. When I was living in an unheated bedsit with mould all over the walls and holes in the windows, she’d come over once a month to collect all my dirty clothes and bedding, take it to her house, wash it and drive it back to me. So she did love me. And that was my mum.

***

Three months after that seemingly insignificant conversation over doughnuts, I was in Bristol attending a theatre conference. I was hanging out with my friend Jessica when I had a panic attack out of the blue, suddenly overwhelmed by the conviction that my pets were dying, that I’d forgotten to leave them water. The feeling was so intense I checked out of the hotel a day early and bought a train ticket home on the spot. My pets were all fine. I shrugged it off. Then I woke up the next morning to a voicemail from my mother’s number and then I heard that man’s voice, the voice that had haunted my adolescence, that I’d had nightmares about for years. And in that moment I knew that she was dead.

I dropped everything and raced to the hospital. I stood in the morgue looking at my mother’s corpse. The first thing I noticed was the apricot floral frilled duvet case not quite covering the industrial black plastic of the body bag. The mortician’s eyebrows rose as I went to leave: “Stay as long as you like.” There was no need. My mum was definitely dead: cold, stiff and rapidly purpling. She didn’t look like my mum any more, and I guess she wasn’t. I didn’t have to be scared for her any more, care for her any more, try to spare her pain any more. And I couldn’t stay even if I wanted to. I had work to do.

This wasn’t my first time at the death rodeo, and this time it was a race. I knew that the first one to the council offices to register the death gets the little piece of green paper that gives permission to take the body. I had to get there before him. I ran all the way to the tube station, arrived at the council office minutes before 5pm and cried my way in. I was gasping and sweating but I got the green paper. I beat him. I won.

The next few weeks passed in a haze. An Olivier-winning playwright invited me to co-write a play with him. It was the biggest thing that had ever happened to me career-wise, and I knew I should be thrilled, but all I could think was, I wish I could tell my mum. For the first time it really hit me that I wouldn’t ever be able to, ever again. I accepted the job and went to work with what turned out to be the best and most nurturing group of people I’ve ever met. It protected me from falling into an abyss of grief. And on one brilliantly sunny afternoon I decided I needed to take action. Somehow I found myself walking down the street I grew up on, admiring the immaculate front gardens, and looking at the parakeets in the trees, with a knife in my pocket. I walked down the driveway and knocked on the door and he answered it.

A memory: I am locked into my bedroom screaming. I’m probably 15. He’s trying to kick the door in. I think he’s going to kill me, hurt me. He vanishes for a while. I don’t know what’s happening. He reappears outside my door, with a smug triumphant tone of voice: “Naomi, I’ve killed your hamster. I put her on the carpet and stomped on her with my boots until she was dead. She’s squished all over the carpets.” I could hear my mother crying and saying: “It’s not true, he’s lying, he’s lying.” He was lying – the hamster was fine. But I moved the hamster cage straight to my bedroom, so I could protect her. Just in case.

If he was shocked to see me that day, given that we’d last laid eyes on each other when I was 16, he didn’t show it. He invited me in and offered me a cup of tea. We both pretended to ignore the utter chaos of the house, what looked like the entire contents strewn all over the carpet, drawers upended and entire bookshelves swept on to the floor. He went into the kitchen to switch on the kettle and I started to search. I found what I was looking for and pocketed it.

My heart stopped when I glanced at the will and my name wasn’t in it.

“So I told the doctors to pull the plug,” he said when he returned, a few minutes later. He didn’t catch me searching the house: I was too quick for him. But later I realised that he intended me to see the will.

He took a sip of his tea then stared at me, smirking. “Don’t worry, I’ll let you come to the funeral. And I’ll let you pick something to remember her by.”

I leaned back, and the point of the knife I’d brought to protect myself jabbed into me. It wasn’t a gang-crime knife; it was part of a kitchen set from John Lewis. A paring knife, intended for fruit and veg. I suddenly felt very, very small. He’d just finished telling me what had happened the night my mother died, or his version at least. That they’d gone to bed in their separate bedrooms, and he’d been woken by moaning. That he had called an ambulance, and that the doctors in A&E had told him that she’d had a stroke and asked him to make a decision. At some point the doctors had told him my mother only had a 20% chance of survival, and that she would be disabled if she lived. The doctors, he told me, had asked him to decide whether they should treat her or not. Maybe he used the word resuscitate. I don’t remember. He explained – bragged – that he’d decided to “pull the plug”. I didn’t give him the satisfaction of asking why he hadn’t called me, why I wasn’t allowed to say goodbye, why the hell it had been his choice as her ex-boyfriend whom she was in the process of evicting from her house and her life (but did he know that? I’ll never know).

The next day I went through my mum’s mobile phone, which I had snuck out of the house along with the will, thinking back to the conversation we’d had over doughnuts a few months earlier, when she’d pressed that yellow solicitor’s card into my hands. She’d told me that he had forced her to make a will leaving him everything, and that she’d managed to sneak out to make a new one in secret, in a way that I think is pretty badass. She’d formed a new group of dog-walking friends he didn’t know existed, that he couldn’t alienate or control her access to. They closed ranks to protect her. My mum lied to him that the dog was ill and then snuck out of the back of the vet’s surgery, letting her friends look after the dog while she popped into the solicitor’s office across the road. She told this solicitor that she had broken up with a live-in boyfriend whom she was afraid of and couldn’t get rid of, and that she wanted to ensure her daughter and her daughter’s inheritance would be safe from him.

I had lost the solicitor’s card, but I used her mobile phone to access her email account, and then her secret email account. I scoured them to find the solicitor’s name. “We’ve been expecting your call,” they told me, when I rang them.

My mum, it turned out, knew that the legal system is more powerful than a knife. He had thrown me out of my own home when I was practically still a child, and there was a poetic justice in being able to return the favour so many years later. It took six months to get him out; in the end he did a midnight flit, leaving the house so trashed all the carpets had to be ripped up and thrown away. In a final act of malice, he’d not only locked the door but bought a heavy-duty padlock just to fuck with me. I only wish I could have seen his face when he discovered there was a second will.

It was so hard to figure out exactly what had happened on the night my mum died. Many months later I talked to her neighbours, who all reported different things. One said they’d heard fighting, but when pressed they couldn’t remember if it was definitely the night she died, or a different one; they said they heard screaming and shouting so often it all blurred into one. The neighbour immediately downstairs said they heard him at 2am (or was it 3am, or 4am?) tearing the house apart, before her body was even cold. No one could agree what time the ambulance arrived. No one could agree if the paramedics had found my mum in her bed, or at the foot of the stairs. No one could agree on anything. But whether there’d been a fight or not, or whether he’d simply beaten her down with so many years of abuse, her body had given up: I felt that he was responsible. Maybe not legally, but morally, at least. And ordering doctors to stop treatment, telling them they could “pull the plug” on a person you are not related to, whose death you believe will financially benefit you, is surely some fiendish Agatha Christie-style form of legal murder.

I went on with my life. I focused on my growing writing career and new friendships and relationships. But I still had questions, so I pushed for an inquest. And, finally, there were answers. “I don’t know what your mother’s former partner told you, but her stroke destroyed her brain,” the A&E doctor who’d been on shift that night told me. “There was no hope of any recovery, and we certainly would never ask any next of kin whether to treat a stroke patient, or ‘pull the plug’.” I felt so stupid because I should have known he was lying. It was the hamster all over again. There’s so much I’ll never know about what happened on the night my mother died. But I’ve stopped looking for answers now. Instead, I’ve decided to be inspired by her strength and resourcefulness; by the way she took back control after years of abuse. By the love that she put out into the world; by the empathy and compassion she never lost. By her creativity and her love of the written word. And by the fact that she never stopped fighting for me.

• Happy Death Club: Essays on Death, Grief and Bereavement Across Cultures by Naomi Westerman is published by 404 Ink. To support the Guardian and Observer, order your copy from guardianbookshop.com.