In the same way that when you are full you can’t imagine being hungry, when you have a best friend you can’t imagine being their enemy.

I was in bed. I had bought a 2m-long phone charger so I could comfortably play Cake Shop 2 while lying down when, bored of my own peace, I went on Instagram. Among the usual DMs – one from a non-profit sunglasses company saying, “We love your account’s vibe”, a man saying, “My dog has drawn you”, and an invite to be on a podcast about DJing – was the message from her.

I zoomed in on her profile picture. It was my best friend from secondary school. The same teenage features, now stretched into an adult’s face, were sitting in a car holding a Chilly’s water bottle, with a smile the same shape as if she were midway through brushing her teeth. The name I’ve chosen to pretend her name is, is “Erin”. I didn’t open the message.

I met her one summer evening by the stone wall that always had teenagers puking or falling over near it. She was smoking, in the dark, with two other girls from the village, their cigarettes glowing like the electric bars on my nan’s old heater. When she clicked her lighter, the flame made her eyes look huge, like moons. But up close I could see this was just the effect of layers of black liquid eyeliner. I said I liked her jacket while looking at my shoes. She said, “Thanks” and called some boys over. Without discussion, we became best friends.

We spent all summer in her baby-pink bedroom making up dances, taking it in turns to say what we thought a “blowjob” was and placing phone books on our lower stomachs to imagine what the weight of another person would feel like there. I knew we would speak every day for the rest of our lives.

So why was she messaging me now, years later? I chewed off a cuticle and opened the message. But instead of reading it, I held the message away from my face like it might go off, and imagined what it said. Maybe she had good news. Maybe she had bad news. Maybe someone had died. Maybe she had died and this was her child or mum hopping on to her account to tell me. Maybe the recent 90s fashion renaissance had triggered in her a wave of nostalgia. I remembered one of her short stories winning a prize at our school eisteddfod: maybe she wanted writing advice. Maybe she wanted help getting verified. Maybe it’s a bot. Maybe she’s selling a table. My phone screen locked and went black. In the screen’s reflection, an adult was staring back at me. For a moment, I forgot I wasn’t 14.

One lunchtime, Erin pointed at a tall boy with red cheeks and said, “That’s Owain. He’s the one I fancy.” Later, Owain came up to me in the library and said, “Do you want to get off with me after school?” But Erin liked him and I hadn’t kissed anyone yet and didn’t know the tongue movements, so I said no. That afternoon, I was in an art lesson, copying an ankh from a book on Egyptian art, when everything changed.

Erin was silent. “Your ankh’s really realistic,” I said. But she didn’t reply. Instead she got up and, with her half-finished drawing, went and sat on the table behind me. Then, she gestured to the other girls to join her. As stools scraped, I sat confused at the centre of a flurry of charcoals and A3 heavyweight paper, and was left on my own. I was stunned by Erin enforcing this strange new power, like the first time I saw an adult put out a candle with their fingers.

Maybe it was a joke I didn’t get. So I just giggled and carried on drawing the ankh. But then I felt Erin point at me and say to her table: “Is she wearing shoulder pads? Why are her shoulders so wide?” Everyone laughed. I didn’t turn round and hoped she’d think I was too engrossed in cross-hatching my ankh to hear her. I swung my legs on the stool with such performative nonchalance that, if the Big Brother body language expert had seen me, she would have said: “This girl is fine.” Then Erin kicked my stool and I turned to her as if I’d forgotten she was there and she said, “Why would Owain like you? Look at your fringe. Who cut it: a bimbo?”

I rested my head in my hand, pressing my hair into my ear, but their laughter and voices still came through the hair barrier. I couldn’t hear the words, but I could feel them in my stomach, like a distant drum beat. I felt my face go red like someone had got the trainer bra and pants from my bedroom and was holding them up, one by one, on the six o’clock news. At the end of the lesson, the teacher looked at my ankh and said that one side was very realistic, but the other side was sloppy.

That night, I couldn’t focus on my homework. I looked out of my bedroom window at every noise that sounded like her bike, thinking she was coming over to say sorry. But she didn’t.

I told my mum what happened. Without stopping moisturising her neck, she said: “You’ll be friends again tomorrow.” I didn’t move. So she turned around, put two creamy hands on my shoulders, smiled lovingly, and said: “Don’t let it get to you.”

Whenever I spoke in class, Erin would do an impression of my lisp and whisper, “Who’s that stupid toddler?” But I didn’t let it get to me. When she said to me in the girls’ changing room, “I wouldn’t wear sleeveless tops if I were you,” the shame made me nauseous, but I didn’t let it get to me. When she said on the bus, “Why is your hair so greasy – can’t you afford shampoo?” my hands tingled with hurt, but I didn’t let it get to me. I grew out my fringe thinking that it would protect me, and I hung out with another sad girl even though I didn’t like her, because it was 1% less bad than being alone. But I didn’t let it get to me.

“Just be patient,” my mum said as she trimmed my fringe over the bathroom sink. “It’ll all be over soon.” At the word “patient” I suddenly had a plan: I would just hold all the pain inside and wait for the bullying to stop. Being patient was something I was good at. When I would pick out a party dress from my mum’s Next catalogue, it would take six weeks to arrive, but the waiting was easy. Because I just knew that once I owned that party dress, I would be so happy. (I still feel this way about dresses now.)

Back in my bedroom, I brought the phone closer to my face and sat up, barely scratching the surface of the slack of the 2m cable. In the old 1m days, I would have been rebounded by the stinginess of the cable, and this move would have been impossible.

I unlocked my phone to reveal the message again. It was only a few lines long. I skimmed its surface, like when you open a letter that informs you whether you have passed or failed something and you look for a word like “delighted” or “unfortunately”. I had a sudden urge to sleep, which always happens when I want to avoid something. My shoulders were hunched up close to my ears and my mouth was dry. I forced myself to start at the beginning of the first sentence and to read it properly.

It ended with a fight. Erin said she wanted to meet me in the local park. I didn’t want to go, but the shame of not going was greater than the fear of going. And the hardest thing to have patience for is your own shame.

There was nothing in Mizz magazine about how to choose an effective weapon, or how to sneak it out of the house while your parents were watching Coronation Street. So I took a golf umbrella from beside the front door.

The fight began. Erin’s weapon of choice was her cherry red Dr Martens with steel toe caps. She tried to kick me and I waved the end of the golf umbrella at her. As her hand came towards my face, I thought, “There’s the hand I put a friendship bracelet on, the hand that made pizzas from scratch in my kitchen with me and my mum, the hand that painted my nails.” As I tried to anticipate the arc of her hand’s movement, we briefly made eye contact. Once, I would stay up all night and look into those eyes, rather than fall asleep for a second, because I wanted to keep talking. Now, those eyes were trying to hurt me.

Eventually a woman and her beagle ran over to intervene. I wondered if we’d be sent to borstal and whether I’d be allowed to ask for separate cells. Erin spat on the ground in front of me and said, “You’re not worth it anyway.” Those were the last words we exchanged.

Until now. Her message said: “I’m sorry I was such a bitch. Can you forgive me?”

“That’s so nice,” I thought. I started typing a reply: “Hey! It’s fine! It was all such a long time ago.” I thought, “This is the right thing to do.” I continued to type: “Of course I forgive you. I never think about it.” And I thought to myself, as women think to themselves hundreds of times a day: “It wasn’t that bad.” When I finished typing, there were red crescents in my palm where I had dug my nails into my hand. I deleted my draft reply.

How bad was it, really? I didn’t wear a sleeveless top in public until I was 32, because of what she said about my shoulders, but it could have been worse: I could have been 35. I only spent a year in speech therapy to get rid of my lisp. And I’ve only mentioned her about once a month in therapy, definitely not every week. How do you calculate the impact another person has had on you? Well, it’s either ruined my life, or it’s fine.

The real shame was not that it happened or that I didn’t stand up for myself. The real shame was that the advice I was given could not attend to the wild recesses of the heart of a teenage girl. So I mistook patience for strength and repression for serenity. It was the wrong solution. But I get to do it differently now.

When the bullying was most intense, I would eat lunch alone in a toilet cubicle and pretend I was an adult looking back on my life and I would be successful and I would be laughing about it. So there was pressure on me now, as the grown woman that I had imagined. The 14-year-old was waiting for me to do something.

Back in my bedroom, half-lit by the light of the phone, I thought about my mum, and her mum, and the long line of women I come from who were all experts at minimising their own pain. Even Erin must have had to minimise her own pain, in order to inflict it on another person. I wondered what she was going through back then.

But I had to do things differently. I had to let it get to me. So I replied on that 14-year-old’s behalf, with the truth. “I don’t forgive you,” I typed. And pressed send.

Katy Wix’s memoir, Delicacy, is published by Headline. Buy it for £9.29 at guardianbookshop.com