When I was three, my pregnant mother died in a car accident. Two years later, my father, an eye surgeon, married his secretary, Isabella. She hated me, though it wasn't personal. She'd have hated any child who was there. She was violent; grabbing me by the hair and slapping my face etc, but her main weapon was emotional abuse. Every single day, no respite. Her energy was remarkable; she had an inexhaustible supply of hatred, expended daily, yet burning fiercely for years and years.

As with most domestic abuse, physical torture was only a part. Hers was a war of attrition: relentless, humiliating, terrorising, degrading, twisted, petty. She destroyed what mattered to me: at six, it was the teddy bear my mother had bought me. Aged 12, it was my diary. I couldn't ever protest or express an emotion. If we crossed a road, she'd dig her nails into my wrist. As I turned 10, she hit upon the idea of making me fat. I was forced to finish every disgusting meal. Breakfast was boiled milk with a thick skin on the top, hot lumpy porridge and a jam sandwich, with butter as thick as cheese. Once I was sick into the porridge, but, so scared of her, I ate it.

The iceberg tip of her loathing was visible to all but because my father chose to ignore it, friends and family were powerless. They occasionally showed disapproval – but knew if they were too bold, she might exclude them, leaving me isolated. At friends' and relatives' houses, I was passionately spoiled as the adults tried to compensate for her treatment with subversive acts of kindness. Every one made an impact. I still think fondly of a cousin, sneaking me a Kit Kat.

People find it hard to imagine the mindset of a child systematically terrorised by an authority figure from the age of five. They ask, "Why don't you stand up to her?" Perhaps if they were in prison, they'd stand up to the sadistic guard. I was lucky, though. I had bursts of freedom. My best friend and I pretended she was a witch, and laughed at her. "I know what you say about me," she once hissed. If others were present – a neighbour, the electrician, the au pair – then I was safe. If we were alone, she did her worst.

She washed my hair by forcing my head under the bath tap so I couldn't breathe, releasing me only when I choked and spluttered for air. Then a blow-dry; burning my scalp with the dryer, yanking out my hair with the brush. She cut my nails so short that they bled. I dreaded swimming day at school because when she stormed into my room (I wasn't allowed to get out of bed by myself until I was 14), I'd have to say, "Please can you get my swimming things ready?" And that would drive her into a rage. Every interaction was rough, angry, impatient, verging on violent.

Cornelia, our old German au pair, recently sent me a transcript of her diary. I love her 18-year-old outrage, so honest and humane at a time when honesty and humanity were in short supply: "I don't think I've heard Isabella say ONE kind word to Chrissie during the five months I've been here! When I left the house today, Chrissie was in her room, crying. I am 99% sure it was because of something Isabella said. And Chrissie, never, never, NEVER answers back! I keep remembering how I was when I was 13-14! Jesus Christ!

"Another thing. Chrissie never makes her own breakfast. It's always a bowl filled to the brim with milk and cereal, a cup of warm milk and a double sandwich. I have made my own breakfast for as long as I can remember! Decided for myself how much I want to eat! Isn't that the normal way? Not in this house. Another astonishing fact: Isabella washes Chrissie's hair! Please! Can't she do that herself? She's almost 14 years old!"

Isabella wielded absolute power and control. When I was 14, she allowed me to attend a cool party with friends, but laid out the clothes I must wear: her old flares from the 60s. Drainpipe jeans were in fashion! Also, my hair was flat with grease as she didn't let me wash it. I hid in the toilet. But we still had family outings; we lived close to the sea, and could walk to the beach. To the outside world, not looking carefully, I was privileged.

As a metaphor, the Cinderella law – the name of proposed changes to the child neglect laws, meaning that mothers and fathers who starve their offspring of love and affection could be criminally prosecuted – is perfectly apt. Isabella made me polish the vast oak floors of our house on my hands and knees. That fairytale gave me hope. My brother Ethan, her son from a previous relationship, was never asked to lift a finger, but I didn't mind doing housework: it satisfied Isabella's need to dominate and humiliate me, which meant a brief drop in her anger level. Her insatiable fury was mentally exhausting. When I stroked a dog in the park, she said, "I don't care about you, but if you catch something and give it to Ethan, I'll kill you for it."

I became a robot. I didn't respond to taunts. I desensitised myself to being hit. Inside, though, I was defiant: that, and my friends, was what saved me. And I was blessed in my little brother Ethan – her son – who was just and brave. Only he, outraged, defended me to her face. I was nine when she stamped into my room, snarling, "If you mess up my things, I'll mess up your things!" and swept my dinky china collection on to the floor (I'd displaced a towel in the airing cupboard). Ethan burst into tears and shouted, "Stop being mean to Chrissie!"

He was six.

There was no pretence at equality, as Cornelia noted: "Isabella talks to Ethan, strokes his hair, makes jokes. Chrissie she barely speaks to, other than [to say] 'Lay the table.' She gives sarcastic answers if Chrissie asks anything. In the photo albums (which I've flipped through when the family's been out!), there are hardly any pictures of Chrissie, but page after page of pictures of Ethan. He's never told off. He leaves food at almost every meal – never a word that he should eat everything up. Poor Chrissie can't even ask for smaller portions, because she should be grateful for what she gets! MY GOD! It must be pure hell."

Chrissie's stepmother told her to put concealer on a facial bruise before her father came home, telling her it was 'healing cream'. Photograph: Alamy

You acclimatise to the heat, but I did suffer from depression in my 20s. Yet I wasn't emotionally destroyed because I had my secret supporters. I always knew it wasn't me. I was traumatised though; I disassociated from my younger self to the extent that I only feel pain now if I imagine my own five-year-old daughter in the same predicament. Children are tiny at five. It was harder for me to accept that my father chose to ignore it because I knew he loved me.

Isabella wasn't rough in front of him and once ordered me to put concealer makeup on a facial bruise ("It's healing cream") before he returned from work. But his refusal to acknowledge her treatment of me, or that our family unity was a charade, gave me no option but to play my part.

On the surface we were a functional family. We visited the zoo. We attended church. Each day was a mental assault course, trying to minimise the threat, attempting not to nudge her simmering, scowling disapproval into explosive rage. Only at school could I relax.

Eventually, at 18, I escaped to college in the US. By the time of her death, 12 years later, Isabella and I had established a workable relationship, based on reimagining the past. Even today, my father persists in the fantasy that my childhood was idyllic. I will never challenge him. No one protected me then, so I feel I have the right to be selfish now in preserving what I call family. I suspect most neglected and abused children remain desperate for their parents' love, long after those adults have forfeited any right to it. I would have worshipped Isabella, had she let me.

Of course we want to help children who suffer sustained and deliberate emotional abuse. But how? The proposal of a Cinderella law sparked outrage, and a lot of jokes about parents being dragged to court for refusing to buy their kid a pony. The Daily Mail columnist Sarah Vine declared emotional abuse "amorphous" and the headline asked: "Will mums now be jailed for banning PlayStation?" Allison Pearson admitted in the Daily Telegraph to "lightly mocking" her son, once, and inquired "Does that count as belittling and abusive?" No, no it doesn't. An attempt to help kids whose childhood is an endurance test in terror is easy to dismiss if you are ignorant as to what emotional abuse actually is.

And yet I'm trying to fathom what a change in the law could have done for me; on what evidence it could have torn either me or her from our respectable, middle-class home. Really, only my father could have protected me, but unless he'd issued me with a bodyguard, I'd have still been as vulnerable as a slug because he was mostly absent. "Marcus gently caresses her shoulder sometimes," wrote the au pair, "but I don't think he's supportive enough. He leaves all child-raising in Isabella's hands."

A few years ago, a relative confessed that she'd considered approaching the authorities, "But what would they have done? Put you in a foster home?"

I wouldn't have wanted to leave my wider family. I don't believe that any official investigation would have led to the police carting off Isabella in handcuffs. I can't imagine how she'd have punished me then, behind closed doors. There was no solution. No good-hearted attempt to flush out the evil would have ended well for me – my only hope was that she'd be struck by lightning.

In the absence of a fairy godmother granting that wish, I adapted and survived. Emotionally abused children learn self-reliance – sadly, we realise that adults are clueless and useless.

Names have been changed