It has been a peculiar and exasperating five years. I’m a writer. I do other things but writing feels like my main reason for being on the planet. Thanks to a triple heart bypass, some underperforming psychiatric medication and long Covid, however, I’ve been unable to write for most of that period. Much of the time it’s been impossible to read as well.

The heart bypass happened in early 2019, three weeks before my last novel, The Porpoise, was published. Consequently, I had to do interviews either reclining on the sofa at home like a poodle-less Barbara Cartland or down the line from Radio Oxford which, after you’ve had your chest opened with a circular saw and have trouble remembering your own phone number, feels like driving way too fast through thick fog.

I was lucky. I knew something was wrong and was able to do something about it before I had a heart attack. When I was out for a run my heart rate would hit a ceiling and I had the peculiar sensation of something constricting my fuel line (this turned out to be a pretty accurate description of my dangerously narrowed cardiac arteries reaching full capacity). My GP and I ruled out several other possibilities, I went for a scan and was whisked into hospital. Because I hadn’t had a heart attack, my physical recovery was relatively swift. I was walking round the ward a couple of days after the procedure and doing gentle runs and cycle rides after a few months. But my brain was porridge. I couldn’t write or read. I woke up every day feeling as if I’d just downed a large glass of red wine or thrown back 4mg of Valium, and not in a pleasurable way. Sometimes it was infuriating, sometimes I was too tired to feel anything.

It’s generally called “pump-head” or “post-perfusion syndrome” – the assumption being that it’s caused by your blood being routed through a heart-lung machine for four-and-a-half hours, though the only perfusionist I spoke to bridled at the idea, which he took as a personal affront, pointing out that the hypothesis was backed up by no hard evidence and that the condition could equally have been caused, for example, by fragments of powdered bone from my sawn sternum becoming lodged in the fine capillaries of my brain. So for a year after the operation I walked and jogged and cycled and grumbled and counted my blessings and tried not to think about capillaries blocked with powdered bone and got very good indeed at the New York Times Spelling Bee, one of the few things my reduced brain was still able to do.

I tried to write tiny things to keep my brain in shape as a kind of mental physiotherapy. I gave myself some of the exercises I gave to students on the Arvon creative writing course, but the resulting paragraphs never contained the promise of something longer, and looking at these sad little malformed orphans was doing nothing for my confidence, so after a while I stopped.

The wisest medical advice came from my GP who said that, while it was a wholly unscientific concept, many of his patients who had gone through chemo or major surgery needed a Magic Year to recover, and indeed almost exactly at the end of those 12 frustrating months came a break in the fog during which I wrote two short stories, one a version of the Minotaur myth set in Elizabethan England and the other a version of the Temptation of St Anthony set at the end of the third century, both a million miles away from my own life. It was like rediscovering the lost ability to time travel.

Then the fog closed over again. I’d been on mood-stabilising drugs for many years and they no longer seemed to be working. Was it Pump-Head Part II or darkness, my old friend? Depressing fog, or the fog of depression? The drugs I was on were the kind your GP is not meant to tweak so I had a long wait to see a psychiatrist. I’d had non-fun experiences on SSRI antidepressants before – I have a vivid memory of being outside the Jardin des Plantes in Paris and being convinced that the Earth was spinning so fast that if I didn’t hang on tight I might be swept up into the void – so the psychiatrist insisted that I sample a buffet of other drugs before we landed on a different SSRI, which suited me just fine. The fog cleared and I wrote another short story.

Then I got Covid for the second time, which morphed into long Covid. Unless it was Pump Head Part III, it being in the nature of brain fogs that they are hard to distinguish from one another. It was not operatically bad, but I couldn’t go on long runs. Short runs left me exhausted for days. I was sleeping nine or 10 hours a night. Most days I couldn’t read, my attention wandering halfway down a page so that I’d forgotten what had happened 10 lines earlier. I certainly couldn’t write. The best description I heard was from a friend of a friend. Before chemo, she said, she had a big table in her head. She could arrange many things on it, see them all at the same time and understand how they related to one another. Now she had a very small table in her head. She could put a couple of things on it, but add another and one of the original things fell off. You need a very big table in your head to write a short story, let alone a novel. You need to see the multiple possibilities which might flow from an initial idea, and you need to see the longer-term consequences of tiny changes. Even a good paragraph is 100 interconnected parts. Change one and the effects ripple through the whole.

* * *

Again, I’m lucky. I have a fantastic family, two of whom are quite costly to maintain but one of whom has a well-paid full-time job, so the bailiffs were not going to be knocking at the door. Plus, once upon a time I wrote a book that is, to my astonishment, still selling. But what do you do when your raison d’être has upped and offed? Some days I felt as if I was in my late 80s and living in a very high-quality care home. The longer it went on, the less hope I had that I would get better. It should have been terrifying. Occasionally it was. Sometimes I’d get angry with myself. Surely I just needed to concentrate, to force myself to get stuff done, to stop being lazy. But that kind of thinking takes way too much energy to sustain for long, so I’d shrug my shoulders and stare out of the window, or spark up the games app.

One of the peculiarities of the fog – and this, I suspect, is what makes it particularly galling for many similarly befogged people – was that it was largely invisible from the outside. I have a borderline pathological need to make sure everyone around me is OK. I might be staring at the walls at home but put me in the presence of other people and I perked up. When I went home and opened a book, however, the words would swim again.

Something similar happened during my weekly shifts as a listening volunteer for the Samaritans. Just as everyone becomes sober as soon as a fire alarm goes off, so the fog would clear as soon as that phone rang three times and I picked it up not knowing who would be at the other end. Would it be someone who was desperately lonely? Would it be someone in the depths of florid psychosis? Would it be someone standing on a motorway bridge? Those shifts were one of the things that got me through. Not just the sense of there-but-for-grace-of-God but because it was nearly four hours every week when I had to think about other people, an enforced holiday from the sludge of my own head and a regular dose of feeling genuinely useful.

Art helped too. For a long time, before writing The Curious Incident of the Dog in the Night-Time, I worked as a magazine illustrator and illustrated my own books for children (I did the illustrations in Curious itself). And I’ve carried on making art of some kind ever since. When I’m all written out I can turn to art and make things that require no words whatsoever. Then, when I start missing language, I can return to writing once more.

When the fog came down I couldn’t paint or draw but, to my surprise, I could make sculptures. Painting and drawing require sustained focus. You’re always toggling between control and looseness. You’re constantly swapping hats – one moment you’re making the picture, the other you’re wondering what it would look like to a critical stranger, then suddenly you’re the artist again. Sculpture – at least the kind of sculpture I was making – can be made in your head. Thanks, perhaps, to an ability inherited from an architect father, I could build a single three-dimensional shape on that small imaginary mental table. I could then build it in cardboard, paper and acrylic paint, simply by following the instructions in my mind’s eye. It was uplifting to look across my workroom and see this expanding gaggle of brightly coloured, abstract shapes which looked as if they came alive at night when no one was watching, and to know that I was still able to add something to the world.

Instagram helped too. Not just as a virtual gallery to share my work, though that has been incredibly useful (don’t count the likes, just listen to the responses of people who really know their onions), but also as an almost limitless pool of information about what’s going on in contemporary art. It was also a way of both interacting with the wider world and keeping my hand in by fashioning a few decent sentences under a picture, whether it was my own sculpture, my excitement about the Kandinskys in the Tate’s Expressionists exhibition, or exhorting everyone to watch We Are Lady Parts.

I’ve got to know people on Instagram, too, who’ve come to feel like friends, some of whom I’ve subsequently met in real life, some of whom I’ve still to meet (Brian and Leslie, I will get to Hawick and say hello to the beagles). Over the last few months the fog has thinned a little. Permanently I hope, though I’m not making any bets. Paradoxically I seem able to write (this article would have been impossible two months ago) even though I still can’t read properly. Halfway down the page I seem to lose grip and drift away. I earnestly hope it comes back. Writing without reading feels wrong, just as talking without listening feels wrong. It’s a conversation, not a lecture or a sermon.

I’m not complaining. I’m running again, up and down the Thames and on Shotover and around my beloved Wytham Woods, and it’s blissful. Perhaps it’s the simple passage of time. Perhaps the long Covid has run its course. Perhaps the effort of grinding up those hills has finally flushed the powdery bone fragment from my brain. Perhaps the planets have finally aligned correctly. I don’t care as long as it continues.

• Dogs and Monsters by Mark Haddon will be published by Chatto & Windus on 29 August. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.