On a spring night when I was 15 the mental image of a naked child entered my head and the corners of my world folded in. I put down my cutlery. My throat was closing over. Dad sat across from me, 10,000 miles away, and Mum was hunting draughts at the window.

Stoned and smiling, my brother sat next to me, resting his elbows on teenage knees too high for the table. He looked sidelong at Mum and Dad to check they weren’t watching as he teased the dog with a tiny piece of meat. She patted a furry paw on his leg and let out a little squeak, and he looked at me for my surefire grin of complicity. I knew it was funny. It was definitely funny. But the giggles didn’t come, this time.

The image flickered again as he popped the lid of the ketchup bottle in and out, before shaking it and pouring a lake on to his plate. I picked some mashed broccoli seeds from the tablecloth as the image flashed brighter and my ribcage tightened – giant insect legs squeezing me for the first time. I rose and said, “Thank you for the meal.” The dog danced around my feet as I reached for the kitchen cupboard where we kept the leash.

The street was dark and cold, and the dog strained against the collar. Someone was burning bracken and the air was mossy. In the wood I couldn’t see my feet, just two iridescent eyes flashing between the trees. I turned the topsoil of my mind for an answer about what the image meant, but the possibilities made me dizzy, and I had to sit on a wall. Beyond the trees, the noise of distant traffic was the noise of everyone else, everywhere, and it frightened me.

The more I tried to stop thinking about the image, the quicker it flickered. I pulled my thighs up to my chest and pressed my eye sockets hard against my knees, breathing hard. When the dog licked my ankle I raised my head and gasped, as if breaking from water. I mouthed the words slowly to the dark, “What if I’m a paedo?” And with that question I was sucked inside my head, where I spent the next decade, fretting at the unanswerable like a fly on a lamp.

I have pure O, or pure OCD, a little-known type of obsessive-compulsive disorder. People with pure O experience repetitive thoughts, doubts and mental images about things such as sex, blasphemy and murder. Needless to say, I don’t feel too “pure” when I’ve woken every morning for a fortnight to the crystalline thought of assholes.

Purely obsessional OCD is so-called because the compulsions are largely invisible, and not often acted out in the more obvious, better-known ways such as cleaning or hand washing. Pretty much everything about pure O is secretive. These are things you’re not even supposed to think about, let alone talk about. How would a teenage boy tell his parents that he thought about having sex with his sister, a thousand times a day? What if you were a mother and you kept having thoughts about drowning your baby in the bath? Or a gay man who kept having thoughts about vaginas when you made love to your husband? How would you begin to talk about it? You’d keep it secret for years; for your whole life, perhaps.

This is why it’s difficult to say how many people have pure O. One estimate puts the figure at 1% of the global population, or 630,000 in the UK alone; but it could be significantly higher, as many people with the condition don’t even realise they have it. Why would they? If a boy was suddenly seized by repetitive thoughts about shagging his sister with, say, the narrow end of an avocado, would he automatically assume he had a neurotic disorder? How could he possibly know that messages were misfiring in his brain and preventing him from dismissing the kind of what-the-fuck thoughts most people shrug off without worry? He wouldn’t. He’d assume he had a deep-rooted personal problem.

In an effort to resolve it, he might Google the meaning behind his thoughts. He might deliberately conjure mental images of his sister while monitoring how he felt: aroused or repulsed? Excited or horrified? He might start ignoring her calls, or give up guacamole for ever. He might spend 10, 16, 20 hours a day in a spiral of rumination and problem solving, trying to figure out what the hell was happening to him.

He wouldn’t understand this yet, because he wouldn’t know that he had pure O, but all these attempts to rid himself of doubt and anxiety would merely be compulsions. And because he was so terrified of someone discovering his shameful obsession with incest (and avocados) he’d strive for normality. Even though the World Health Organisation considers OCD one of the top 10 most debilitating conditions in terms of quality of life, not a soul would know.

After my first panic attack on that spring night in the wood, my mind started spinning. Am I a paedophile? This was the big, pressing question of my adolescence, bigger than the Kickers-or-Pods question, bigger, even, than the Keanu-or-Leo question.

In a bid to answer it and purge the anxiety, I began to dissect my memory for clues about my identity. I analysed every pretend kiss and cuddle I’d had at sleepovers; when my friends and I had re-enacted Neighbours weddings, pressing our faces together and giggling at the “kiss the bride” bit. Or when we’d renamed Barbie and Ken as Fanny and Dick and made them “make babies” in a shoebox. All these filthy sparkles of a child’s imagination were twisted into something threatening, because they seemed to support my obsessive fears about my capacity for depravity.

By the time I sat my GCSEs, the images and thoughts were flashing up like searchlights in my face, 24/7. During long exams, every second stroke of my pen marked the flicker of some forbidden obscenity in my brain. Sometimes I got up in the night and had five seconds of forgetfulness. But by the time I’d stepped blinking into the bathroom, the thoughts had always caught up. The next day there’d be teeth marks in the toilet roll where I’d stopped myself from screaming.

Church was the worst. There was the penitential rite, the confession and absolution. Mea culpa. My fault. There I was, every week, a child, saying the words and trembling: “I confess to almighty God, and to you, my brothers and sisters, that I have greatly sinned, in my thoughts and in my words”. I was at fault because God had said so. Barbie and Ken had been my fault, kiss-the-bride had been my fault. My thoughts, even, my unstoppable thoughts – they, too, were my fault.

So I’d lie in bed on Sunday nights murmuring that line over and over. I have greatly sinned in my thoughts; I have greatly sinned in my thoughts. And I’d slip into sleep on the damp pillow, trying to focus on the sound of my parents’ heavy sleepbreathing in the next room, or on the ceiling’s fluorescent stars; on anything that was outside of me.

I wasn’t always obsessed with paedophilia, though. As is common with OCD, the theme of my obsessions changed, and I was 17 when I first noticed the inexplicable new thoughts creeping in. My friends and I were playing bingo in the old Dudley hippodrome and I started seeing images of their tits in my head. I tried not to think about what I’d seen. But each time I pressed the soft ball of the red bingo marker on to the paper, I saw the images again; I couldn’t look up from the page.

Back at home that night, I sat down to watch the most innocuous TV programme I could find – Ray Mears – hoping to snatch a few minutes’ respite from the thoughts. But as the camera panned down across a cliff face, each crevice became a startlingly detailed vagina. I froze and spat a mouthful of crème caramel back into the plastic pot. “Am I gay?” I whispered.

Within minutes the question had taken on a pathological urgency, and I was scouring my memory for an answer. Peeking at the breastfeeding women outside nursery, all those years ago. Did that mean I was gay? Kiss-the-bride? From then on, every minute of every day, I wasn’t seeing naked children, I was seeing naked everyone, compelled to figure out which thoughts turned me on the most. The dinner lady or the headmaster? The lollipop lady or the policeman? Cherie Blair or Tony Blair?

I was meticulous. I’d buy Attitude and Diva, spread them out on my bed and sit there waiting for an answer to rise up from the centrefolds. At university in Leeds I would “try out” gayness some days, bouncing to campus like Pinocchio to school; other days I’d be unequivocally straight. I’d describe my gay thoughts to my friends and use their reactions to gauge the plausibility of my homosexuality. I’d browse profiles on lesbian dating sites, trying to imagine myself kissing each stranger’s face. I’d oscillate between these periods of intense immersion in sexual content and periods of avoidance, during which I wouldn’t watch TV or read the paper, to starve the sex out of my head, the anxiety from my chest.

And so went the next seven years of my life, or my “life”, I should say. Because when the pure O exploded, my life grew inverted commas and flew away. All that was left was an effigy of a young woman and a neon pink MySpace profile.

Sexual orientation doubts are common among straight and gay sufferers of pure O, and the obsession has an extra sting its tail. Because the mental anguish and experimentation involved so closely resemble a coming out process, they often get misinterpreted as such by sufferers, and by those around them. I certainly got stung, and the confusion was dizzying. I had no reason, moral or personal, to be afraid. I was ardently pro gay rights, and I always thought lesbianism was totally hot. So why was I so terrified?

I didn’t understand that I had the “doubting disease”, as OCD is otherwise known. I didn’t know that it was the uncertainty itself that was frightening, the possibility that I might never know my “true identity”. Neither did I understand that my soul-searching behaviour was actually making my thoughts worse. I was wholly ignorant of the bitter irony that in constantly seeking certainty, pure O-ers render themselves more uncertain. As OCD expert Dr Steven Phillipson writes in Thinking the Unthinkable, “The tremendous effort one puts into escaping the unwanted thoughts or preventing their recurrence (eg hiding knives), in effect, reinforces its importance to the non-conscious brain and, thereby, feeds the vicious cycle… Becoming upset over a thought places a mental marker on it and increases the likelihood of the thought recurring.”

I didn’t understand that the only way to treat pure O is to stop acting out compulsions and break the vicious cycle. So it spun ceaselessly under every moment, churning up jobs and relationships. On the first day of a placement at the BBC, I hid in the toilets because the whole news room had appeared to me naked. I split up with a boyfriend because every time I kissed him I saw the Ray Mears cliff face in his eyes. My memories of that time are Pure O memories.

By 20, I believed I was locked in an irrecoverable sexual identity crisis. I’d quit uni and was contemplating suicide daily. Embodying the rank irrationality at the heart of OCD, I would rather have died than lived indefinitely with the doubt. Then, one day, when I was Googling the meaning behind the comedically graphic sexual content in my dreams, I landed on a Wikipedia page about pure O, and, hardly able to breathe, gasped as I read my symptoms. Repetitive distressing thoughts? Check. Thoughts antithetical to desires? Check. Extreme anxiety? Inability to dismiss thoughts? Constant rumination? Check. Check. Check. This was it. The proof that I was neither a closet case nor a homophobe, that I’d never been a paedophile. I was just ill. I had a diagnosis!

I consumed the information voraciously. Pure O commonly starts between early adolescence and your mid-twenties. Pure O thoughts are referred to as “spikes” by the OCD community. Spikes: of course! They do spike. Pure O is often combined with major depression and other anxiety disorders. The condition is widely mistreated due to a lack of awareness and training in the medical profession.

After a few days I knew some bits of the Wikipedia article by heart, and started reciting them as rebuffs to my obsessions. Every time I had an intrusive thought I’d shout it down with the retort: “It’s not me, it’s my OCD.” My brain, finally convinced of the truth, would surely cease its indecision. For about a week I thought it had.

But soon the thoughts and images flared up again, and the insect in my chest tightened its legs around me, tighter than before. Because no matter how much you reason with OCD, it always finds a loophole and redoubles its ferocity. Soon I was back online, reading the same articles for my next fix, until I once again reached a precarious sense of certainty about who I was.

Eventually I went to the doctor with my self-diagnosis. First I got referred for person-centred therapy, in which a counsellor tried to get me to come to terms with my latent homosexuality. Then I went for psychodynamic therapy, where I was diagnosed with pure O before being prompted to explore and analyse the route of my thoughts, à la Freud – effectively encouraging me to engage in compulsive soul searching. This was the wrong approach: analysis only made my obsessive thoughts more deeply entrenched.

Then, after a six-month wait, I received cognitive restructuring therapy, which used rationalisation to prove that my thoughts couldn’t be true, based on x, y, z evidence. While highly effective in the treatment of depression and some other anxiety disorders, cognitive restructuring of obsessive compulsive thoughts is woefully detrimental, for the cyclical rumination it encourages. You cannot out-logic OCD.

Sufferers of OCD will go for up to 10 years without effective treatment. I met a few in group therapy: a father terrified he might abuse his children, a young girl convinced she might burn the house down, a woman who thought she would run people over if she got behind the wheel. They shared my story: lifetimes of secrecy and ruinous therapy. Enter an online pure O forum and you’ll hear voices screaming as if from under ice, spewing their obsessions onto the page or offering kind-hearted but disastrous advice to others. Week on week, in this country and all over the world, misguided therapists are systematically making these individuals’ OCD worse.

After four years in Leeds, I moved to London. I met a boy and fell deeply in love. I drove across the world in a double-decker bus. I met Jake Gyllenhaal on a music video shoot and watched his face melt into a chubby vagina in my vision. I sat in the Melbourne mansion belonging to the founders of Lonely Planet, imagining them fucking across the patio. I nearly overdosed.

In truth, I owe much to Gyllenhaal’s vagina face, because the suicidal spiral it prompted was the necessary catalyst for my seeking private therapy. I chose an OCD specialist at a world-leading centre for the treatment of anxiety disorders in New York. Every Monday for a year I had a 45-minute session of exposure and response prevention (ERP) therapy on Skype, in which I was exposed to sexual images of gradually increasing explicitness. I had to let my thoughts wash over me unresisted, while my anxiety shouted and screamed and had me ripping my cuticles in strips from my thumbs.

I was a studious patient, diligently watching porn three times a day for months and months. I watched so much porn I could identify the production company by the luxuriance of pubic muffs or lack thereof. Eventually, thanks to an awe-inspiring phenomenon called neuroplasticity – which means we can bring about physical changes in our brains’ neural pathways and synapses by changing our behaviour – I began to get used to the anxiety and to relax my need for an answer.

While recovery rates are excellent with the right therapy, there is no neat panacea for pure O, and the final act of stoicism for anyone post-therapy is accepting the possibility of having the condition for ever – while conversely accepting that their obsessions may, in fact, reflect reality. I wrote every word of this article reminding myself that it might be a cover-up for who I really am. It has been an incredibly liberating experience.

Since I was 15, pure O has underscored everything I’ve done, and I may never be without it. But in a small way, I’ve come to love it for the far-reaching wisdoms lurking within its fetid little heart. When we try to fight our thoughts, pure O shows, we only make them stronger. It is only when we give ourselves the freedom to be uncertain and insecure, that we reach a deeper sense of who we are.

In the past four months since I finished therapy, there have been moments when the pure O has lifted, imperceptibly, like rising light, and I’ve had no thoughts in my mind; felt nothing but the quiet joy of concentration or the shimmer of my boyfriend’s touch. If it wasn’t for the comparative cacophony of pure O, I wonder, would these moments feel so impossibly beautiful in their sheer, simple unthinkingness?

• Rose Bretécher is a pseudonym.