In a world where, for many twenty- and thirtysomethings, just about every marker of adulthood – home ownership, grownup holidays, quality furniture – is out of reach, marriage, says Maggie in Monica Heisey’s new novel, is the “only hallmark of what we think of as an ‘adult life’ that’s still accessible”. As long as you can find someone to go into it with, that is. What happens, then, if you get married and then divorce while still young? Have you failed at adulthood? This is what Heisey writes about in her debut novel, Really Good, Actually, which has zoomed up the bestseller lists since its publication last month.

Maggie and Jon fell in love at university, moved in together because neither could afford to live alone, and got a cat. Marriage seemed the next step. Then, at 28, Maggie found herself divorced; the same age Heisey was when her own marriage ended.

Jon rarely appears in the novel, which charts the aftermath – poignant, sad and funny – over a year, of what it is like to be broke, alone, heartbroken and divorced when barely any of your friends are even engaged. “It is hard to explain,” thinks Maggie, “exactly how mortifying it is to have had a wedding when your marriage ends almost instantly thereafter.”

Heisey grew up in Toronto, where the book is set, but now lives in London. We meet at an outdoor cafe in a churchyard, surrounded by deafening birdsong and silent tombs. She has been a standup comedian, a screenwriter (she was in the writers’ room on the hugely successful Canadian sitcom Schitt’s Creek), and has published a book of essays, but this is her first novel. She started it just before the pandemic hit; the subsequent disappearance of much of her work gave her the chance to get it done.

Maggie and Heisey share superficial similarities. They are both red-haired, formerly cat-owning, and, crucially, divorced. Heisey, now 34, had the idea for the novel when she was going through her divorce, but she held off from writing about it. “I didn’t want to be writing from a place of therapy,” she says. “I wanted to go to therapy for therapy, and write as a creative exercise. But I also didn’t want to wait so long – I don’t particularly want to be talking about it a year from now.”

Did it bring back tricky emotions? “I had pretty much talked it to death in therapy. I drew on my own feelings, but almost everything in the book that happens is invented, so it wasn’t like I was reliving my most painful memories.”

Maggie is a mess, in a way that Heisey – thanks, largely, to therapy – wasn’t. She shops online too much, and overshares on social media. “I’m fascinated with internet use where people are joking through what seem to be fairly serious instances of pain in their lives. A classic thing is women talking about bad dates online, and they’re writing in a joking tone, but the content is not actually funny. I’m interested in that impulse, and why it might feel good to do that, and what we get from owning those stories.”

Maggie, says Heisey, is someone “whose whole – sorry to use the word ‘journey’ – is one of running from the internal and really trying to skip over the part where it hurts, or feels embarrassing. She’s just dealing with it all externally, in pursuit of a glow-up or whatever, and she’s not interested in doing the more unpleasant, internal work of sitting and having a broken heart.”

There is pressure to show people you are fine, even when you’re not, but also to reinvent yourself as a better person, to show your ex what they are missing. Just about every breakup film has a makeover montage, says Heisey with a laugh.

Did she fall into that trap? “As a child of the 90s and 00s, I don’t know of anyone who came out of that period with totally healthy relationships to body image and food,” she says. “Even if you’ve done a ton of working on yourself, and you can really see what’s going on, in periods where you’re down like that, it can be very easy to pick those scabs.” It’s dreadful to torment your body into fitting societal beauty standards, but, she says with a pained smile: “If you give in to it, the response from people around you is probably positive, which is quite grim.”

She wanted to write the book that might have helped her when she was going through it. “I felt the normal feelings of failure and shame when a long-term relationship ends, but also very isolated,” she says. She was the only person she knew, other than people of her parents’ generation, who was divorced. “When I looked for books, or films or TV to make me feel a little less on my own, everything was about people many decades older and many income brackets above me, which was further isolating.”

In the book, Maggie reflects that divorce films usually have a “beautiful middle-aged Diane” – Lane or Keaton – “who is her own boss and knows about the good kind of wine”. Or, adds Heisey, they are Kramer vs Kramer-style heavy dramas. But when you don’t have children or financial assets to fight over, “it feels like you’re sort of fumbling around in the dark.” Who gets the cat? (Heisey’s ex-husband did.) Or tells the landlord?

“Even in the depths of it, there was something kind of funny about it,” she says. “Heartbreak is this weird thing where it feels like the end of the world. It is emotionally devastating. But equally, it’s a fairly mundane experience. Not an everyday experience but a multiple-times-in-your-life experience, and you know that you’re going to get through it.” She compares it to food poisoning. “There’s a period where you genuinely feel like you live on the bathroom floor now, but you know, because you’ve done it before, it’s going to pass. I thought there was something funny in that.”

Heisey was a bookish child, who grew up with her twin sister and a younger sister; her father was a lawyer, and her mother worked for the government. “My parents were obsessed with us reading classic books, which I found a little dry, but when we found funny ones, it was a sweet spot for their interests and mine, like Charles Dickens and Jane Austen.” Heisey loved British writers, PG Wodehouse, Douglas Adams, and characters like Adrian Mole. At her performing arts high school, she wrote plays and, she says with a grimace, performed in the improv troupe. “Worse than that, we thought the improv team was cool.”

She came to the UK in 2010 to do an MA in early modern literature, and started doing standup comedy “to meet people”. She didn’t find it too intimidating? “I think you guys have really generous audiences. The Edinburgh fringe really trains up audiences in addition to comedians, and you can have these crazy mixed bills where there’s someone doing comedy poems, and a double act, and someone’s singing and everyone can kind of roll with it.” She did perform at Edinburgh, roped in by her friend, the comedian Cariad Lloyd. After a few years on a student visa, she says: “I ran out of options. I was a freelance writer at the start of my career and the government was like, ‘Please leave.’”

She had been in a long-distance relationship with the man who is now her ex-husband, and when she went back to Toronto, they got married. It did feel grownup, she says, though, looking back, the other trappings of adult life didn’t magically materialise. “We got married and then we took the streetcar back to our house, our roommate gave us a beer.”

There was, however, some power in this state-sanctioned new level. She remembers someone telling her that their older relatives started treating them with more respect after marriage, and Heisey found the same. “There’s no thrill like telling your landlord, ‘My husband and I would like …’,” says Heisey with a smile. “And then also you can’t discount being in love, thinking it’s romantic. It is romantic.”

Heisey had been working on Schitt’s Creek when she got divorced, but moved back to the UK. “I was very lucky that I have a group of friends here who I really love, and also nobody in London knows me as someone’s ex-wife. Toronto is a really small place. In a lot of ways, the book is like a nightmare version of what it would have been like to get divorced and stay in Toronto.”

She can’t help thinking that if it were normal to have lavish 30th birthday parties, “where you spend thousands of dollars on your outfit, and everyone agrees to go because of the power of this institution, and photographers came and your friends gave speeches about how you’re a beautiful genius, we might have fewer weddings”. Ceremony, she says, “is very appealing. It’s nice to feel like you’re gathering your community to celebrate a milestone. Does it have to be marriage? I don’t know.”

Even though marriage is in decline, and we are all supposed to be wise to the trappings of patriarchy, it does still have a cultural hold, particularly if you are a woman. “People start asking if you’re going to get married again, instantly. They’re obsessed, which I found very odd. Like, let me sign the divorce papers first.” Heisey is in a new relationship now, and says: “People seem to think that’s the bow on my story. To me, working really hard to process something and writing a book about it feels like a much bigger achievement. But people are always so psyched to hear that you’ve found someone else. And then you’re right back into: ‘Do you think you’d ever get married again?’”

At some point soon, Heisey probably won’t talk about divorce so much (though a TV adaptation of the book is planned). A comedy she has written for Sky is out later this year, she is on the writing team of a US show at the moment, and is about to start writing another novel. “I’m interested in writing something that’s a little less connected to my own experience,” she says. Her divorce seems very much in the past, which must be comforting to anyone in the midst of it. “I was really sad,” she says. “I meant what I said when we got married, I really wanted to try [to stay together]. But a failure doesn’t need to make you feel terrible. There’s lots of failure in life and I think weathering it with a bit of grace and resilience is probably a very important skill to develop.”

The other day, a woman recognised Heisey in a coffee shop and came up to her. “She said: ‘I just have to tell you, I left my husband last week.’ She seemed really excited but she also just really wanted to give me the info, like, here’s a safe person to tell. I think it’s all-consuming.” Heisey remembered her own isolation. “It’s a very lonely time so if the book makes people feel less lonely during that time, that would be great.”

• Really Good, Actually by Monica Heisey is published by 4th Estate. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply

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