I didn't realise how important being bright was to me until I had a daughter who wasn't. I was brought up by searingly intelligent parents, both complicated, funny, intellectual. They bonded over cryptic crosswords and were contemptuous of tabloid-reading mouth-breathers.

I grew up believing that having an incisive wit and split-second recall of arcane facts is more important than being kind or compassionate. I modelled myself on Dorothy Parker and admired her brand of effortlessly cruel humour.

If someone was bright, I warmed to them. If someone was not, I yawned. I assumed that my child would share my feelings. No, I assumed that she would be like me. The first signs were good. In the womb, Bella was constantly kicking and hiccupping, as if she yearned to be out there, engaging the world in a lively debate. And when she arrived she was restless, with a nervous energy that was alarming. My father declared her "very alert" and I glowed with pride, even though she cried endlessly. "She is too clever to just nod off by herself," I reasoned as she screamed through the night.

But as she entered the toddler years, this agitated, highly emotional baby didn't show any signs of developing the cerebral abilities you'd assume came with being alert. She was slow to talk; while friends' children could string together three words – "Wan go slide" – Bella would just utter "slide". All the other milestones found her trailing behind her peers: potty-training, dressing herself, identifying colours. She'd reach them eventually, but always months behind her contemporaries.

It's not helpful to compare, I'd tell myself. Celebrate her for herself. But when you're a new parent, it's impossible not to measure your child against other children.

I searched for reasons. She had a long and traumatic birth – had it left her with slight brain damage? Did all that crying in the first year frazzle her synapses?

I grew weary of the competitive mums at the playgroups with their "stealth boasting". "I'm so cross with Saffron," one mum said. "She's taught herself all her letters – she'll be so bored at school." I'd get a sick feeling when even the sweetest of friends would mention her child's progress, as I hated having airily to reply that, "No, Bella isn't doing that yet".

I felt protective of Bella – she didn't know she wasn't measuring up – but also disappointed. I desperately wanted her to be as good as, if not better, than her friends.

I'd try to "hot-house" her with educational toys, but she found anything but the most basic games baffling. She loved to play with dollies and teddies and grew edgy around anything challenging such as jigsaws – she just couldn't grasp how to tackle them and didn't like me trying to help her.

When she started at infants school, I was hoping that somehow things would just click for her; that she would respond to her teacher differently from how she responded to me.

Bella came home with a work book. We'd go through it together, and she'd carefully draw around the letters I'd outlined in dots for her. But turn the page and the lesson would vanish from her mind. By the end of the school year, the alphabet was still a meaningless set of squiggles to her. I wondered about dyslexia, but her teacher was unconcerned. "She's one of the youngest in the class. Don't worry – she'll catch up," she said. I felt that she didn't care.

Her dad's spelling is dubious and his memory for names and dates is laughable. It must be his fault, I reasoned, glad to have something to pin the blame on to. He wasn't concerned, saying that he needed extra help at school, but seeing as he went on to university and into a profession, it didn't really affect him in the long term.

Comforted by that, I relaxed a little and tried not to be so anxious about Bella. I learned to resist peeking into her friend's bookbags, to see how far they had got through the school's graded library. It only brought heartache when I saw they were light years ahead of Bella. I focused instead on her positive traits: her friendliness and sensitivity. I imagined the cleverer kids gloomily grinding away in a boring job while Bella was surrounded by laughing friends.

But a new school year brought a new teacher, one who was less laid-back. She had noticed that Bella often daydreamed in class and didn't seem to be focused on what was going on, and wondered if there was an underlying cause. I was heartened by her hands-on approach, and gladly agreed to have Bella's eyes and hearing tested. Both tests came back normal. But I already knew her learning delay wasn't due to her eyesight or hearing. It was about cognition.

By the time she was six, she was far behind her classmates – evident by seeing all their work up on the wall in class – and had developed a block about anything remotely academic, because it was associated with stress and failure. To be faced every day with impenetrable squiggles that she couldn't decipher was miserable. The teacher encouraged me to practise reading and writing every night with her, but every letter was a struggle and every session ended in tears, tantrums and dejection. Being a good middle-class mum, I didn't give up. I researched different approaches, turned each exercise into a game, used reward charts and bribery. We had so many stickers that they papered the walls. But ultimately she was resistant to reading and would do anything to avoid it, including sliding off the chair like an eel.

An educational psychologist was called in at school, and declared that although Bella was behind, she was still progressing, just at her own pace. I asked about dyslexia, but they didn't test for it until she was in year three, two years away.

I voiced my concern to Bella's teacher that if she was dyslexic, it seemed a long time to have to endure without any support, but she thought differently. She wasn't showing the classic characteristics of it and anyway, "It may just be that Bella's talents lie elsewhere. Just because you are academic doesn't mean she will be." I instantly felt she had consigned Bella to the slush pile.

Bella looked like me, I so wanted her to be a mini-me, and it was deeply ingrained in me that being clever was important. But while I felt defensive on behalf of Bella, shamefully, I also wanted to distance myself from her, to splutter that I had learned to read before school; I love reading, I'm a writer. It wasn't my fault.

Bella was put into a special class for some subjects, so she had more one-to-one support. I had to sign a consent form for this, and felt mortified. It only served to heighten my awareness that she was not as bright as her classmates, and that I wished she was.

I confided in some friends that Bella wasn't doing well at school and they batted away my concerns with platitudes: "Oh, Bella's lovely. She's so creative, it doesn't matter about that." I think it made them uncomfortable. What could they say?

I asked the teacher whether Bella would always need to be in the euphemistically named "sunshine room". "I just wish all the children got the attention that Bella is getting," she replied. "It's a wonderful way of learning." In other words, yes.

Now that Bella is in year two, her class has been streamed. At the start of term, the parents were brought in to be told what was involved. One mum demanded to know whether there was a top, middle and bottom stream. "No, no, no," they replied. "They are only there to support how the child learns."

I'm no fool. Bella is in the bottom stream. All her friends are in the middle and top streams. Sadly, the only thing the streaming has taught her is that she is behind, as she recently confided while upset. My heart broke for her. All this time I had been stewing in my own frustration and disappointment and anxiety about Bella's lack of progress, I had been comforted by the fact that she was oblivious to it. I had made sure that I told her every day how wonderful she was, despite my inner worries. Her being aware of it made it doubly sad. One ray of hope, though, is that she now sees the point in doing her homework and has discovered an inner resolve. I left her last parents' evening feeling optimistic. She's on the brink of turning a corner, the teacher said.

Maybe she will, maybe she won't. Maybe she'll grow up loving the cryptic crossword or maybe she'll be happier with word searches. I have set so much store on being clever that it may take a while for me to come to terms with this, but it's a lesson I need to learn.