“If you’re going to reject me, then reject me,” I said. I was deep in the bowels of Leicester University, being interviewed for a place at medical school. I was 35, a fact the learned professor interviewing me returned to again and again. How would I cope with the workload? Would the four hours’ driving each day prove too much? How would I support myself through my studies? Concerns that travelled through my own mind. Unlike the questions I asked myself, though, the queries in that interview room were all prefixed with “at your age”. I didn’t see my age as a problem, and eventually I told him so.

“Reject me for the hundreds of reasons you reject people,” I continued, “but don’t reject me because of my date of birth. Your date of birth should be a bit like your National Insurance number. You need it occasionally, to fill in a form, but otherwise why not keep it at the back of a drawer and forget about it?”

Two weeks later, the offer of a place dropped through the letter box. The only other time I saw that professor was at my graduation, and I thanked him for giving me a chance. I didn’t think for a second he’d remember me. He did. “Each year, I pick an outsider,” he told me. “A wild card. That year, the wild card was you.”

I certainly met the criteria for a “wild card”. I had left school at 15, clutching my one O-level in French, but I always knew I’d go back into education at some point, it was just a question of when and how. I did a variety of jobs before I decided to return: I waited tables, I pulled pints, I was a secretary, a pizza-delivery person, a kennel maid. But during all that time, medicine was my only goal. A lifelong interest in mental health meant that in order to fulfil my ambition of becoming a psychiatrist, I first had to qualify as a doctor.

All of the professor’s concerns were valid. The workload was astronomical. The driving each day became almost unbearable. The financial burden was so great there were times I had to choose between a cup of tea and a packet of crisps, because I couldn’t afford both. The most challenging aspect of all, though, wasn’t any of these; it was the small pieces of other people’s lives that passed by me each day. Being an audience to the very lowest, most vulnerable moments of someone’s existence. Not just once. Again and again. Pinballing from one crisis to the next, without ever having time to process any of them. I called them my Kodak moments and I took them home with me every night. I collected so many of them during my training, I soon began to wonder if I was cut from the right cloth to practise medicine after all.

I overcame my fear of cadavers by choosing to attend post-mortems

My first was waiting for me under a cloth in the dissection room, a week after starting my journey to become a doctor. It was the aroma, more than anything, that threw me: a unique blend of chemistry lab and death. The strange rubbery smell of preservative. I stood in the room and stared at the blue cloth covering the “cadaver” we’d be working on during our time as students – and I thought of the last dead person I had seen, a few months earlier. I had watched my mother say goodbye to my father on a watery February morning, surrounded by equipment: the hoists and the commode, the bottles of Oramorph and the monitors all crowded into our front room, trying very hard to blend in with the furniture of an ordinary life.

When a doctor sits across a table from you in a consultation room, you imagine they are somehow swept clean of their own reference points: they are unaffected by memory, or by difficult emotion, or by fracture lines caused by lives lived outside that encounter. An assumption. A necessary fallacy. Because all I could think about, waiting in that dissection room and staring at the blue sheet, was my dad, and I had to leave the room before the dissection had even begun.

I went on to overcome my fear of cadavers by volunteering to attend post-mortems and by doing so saw dissections at their most useful. Even so, the Kodak moments managed to find me. Looking back, my reactions to these moments were red flags snapping in the breeze, but it wasn’t until I qualified as a doctor, and we were thrown into an NHS that bends and breaks with the demands placed upon it, that their true extent became apparent.

All the way through medical school, you are spurred on by the idea of what kind of doctor you want to become. You don’t dream of prizes and awards, but quieter things: having time for your patients, being able to explain a treatment to someone, making a difference.

Instead of being this doctor, you watch patients drift past each day who are clearly unsure and afraid, but there is nothing you can do. Relatives wait for reassurance, but go home empty-handed. Waiting lists are full. Clinics are crowded. Time, money and hope all run dry. You quickly realise you can never be the doctor you wanted to become: the system simply won’t allow it. Because of this, I began to unravel. I hardly slept. Barely ate. I’d wake each morning and reach for yesterday’s clothes. I lost weight. I lost hope. My physical symptoms made my GP query if I had bowel cancer and put me on an urgent referral. I didn’t have bowel cancer, though. I had burnout. An unlikely phrase, because it implies the effects are loud and obvious, raging like a fire for everyone else to see. But most burnout goes unnoticed – even, sometimes, by the one who is burning.

My knack of absorbing small details became my strongest suit

The crisis point was when I met a patient called Gill. We were exactly the same age, almost to the day, and we had grown up with the same song lyrics and the same posters on our walls. The only difference between us was that Gill had terminal breast cancer and I didn’t. I sat with Gill and her elderly parents as she died and I couldn’t stop myself from crying. I apologised for my unprofessionalism, but Gill’s mother said it was a huge comfort to know how much Gill meant to me.

Afterwards, I sat in my car and wondered what I should do. I couldn’t face the hell of going back into the hospital, but I couldn’t go home and admit failure. Yet the thought of making such a devastating experience even a tiny bit easier for Gill’s parents gave me a spark of hope. Perhaps I was a good doctor after all.

It was working in psychiatry that saved me. Even on the first day, I knew I had found my landscape. I’d spent the previous two years believing I was too flawed, too wild to be a wild card. Working in mental health, I began to realise that my flaws were my greatest assets. Talking with patients was no longer an indulgence, but a requirement. The knack of noticing and absorbing small detail became my strongest suit, and it was only now that I truly felt I had the time to make a difference to my patients.

I am often asked about how it feels to be a “wild card”. I usually answer that the world – and medicine – needs more wild cards. What I have come to believe, though, is that we are all wild cards. We are all “unbelongers”. Each of us is just searching for the right landscape in which we are able to fit, and use the unique abilities we possess to our best advantage. As a society, we value some skills above others. As a doctor, I was never bleeped to insert a difficult cannula or assist with a chest drain. I was, however, bleeped if there was a needle-phobic patient who needed blood taking, or someone at the MRI scanner who was having a panic attack due to claustrophobia. Until I reached psychiatry, though, these skills were never considered worthy.

We are quick to identify flaws, both in others and in ourselves, and we always seem eager to highlight our perceived failings and inadequacies. When I took my own “flaws” into psychiatry, the flaws that nearly destroyed me, and when I began to understand how truly valuable they were, I also realised something else – the quality that almost breaks you, the thing that nearly causes your downfall, may also turn out to be the very same thing that mends you in the end.

Breaking and Mending by Joanna Cannon (Profile Books, £12.99) is available for £11.43 from guardianbookshop.com