Growing pains: Hannah with her parents, Vincent and Beth Johnsson / RICHARD MILDENHALL

She was as alert and curious as any toddler. Then she began to lose all the skills she'd learned. Hannah's mother, Beth Johnsson, describes the special heartbreak of living with Rett Syndrome

The most perfect days of my life were the three days after Hannah was born. My husband Vincent and I were in Sweden, where they do things a bit differently. There was a communal kitchen in the maternity ward, which had the best hot chocolate ever. Midwives were on hand, but no one came to bother us, so we were both in one double room for three days with Hannah. We just lay on the bed and looked at her, saying: "She's amazing." It was beautiful.

We soon moved back to England to be near my family and then settled in a quiet cul-de-sac in Sutton. For the first 18 months, Hannah was alert and curious, she seemed just like any other child, except for a few tiny little signs: I had trouble weaning her and she would gag when I was feeding her. I was so scared of her choking. She also laughed when she saw another child cry. Looking back, you can see that's not normal, but at the time, all the professionals were saying that children develop differently and you listen to that because you don't want to believe that anything is wrong.

At 20 months, she started pulling her hair out in clumps. That's when we knew something was really wrong. It was just out of frustration – and she has a lot of sensory issues – but it was alarming. We used to be able to pile her hair up on her head and it would all fall down in beautiful curls. We researched and researched, but the only thing you can do with a child who is pulling out their hair is to shave it off. I took her to the hairdresser and watched it all fall to the floor. I was upset at the time, but now I think that was nothing.

While Hannah was being tested to find out if anything was wrong, I spent months questioning whether I had caused her problems somehow.

I used to think: "What if it's me? What if there's some genetic condition that I didn't know about and how am I going to live with it? What if it's Vincent – how will I forgive him?" The diagnosis of Rett syndrome takes all that away; it's a spontaneous mutation. It's not hereditary. It happens to one in 12,000 people and it happened to us.

Hannah was three years old when the geneticist told us she had Rett. I remember leaving the hospital in pieces and knowing that things would never be the same, but I still thought she might learn to read and write. I thought of it as a severe learning disability, but it isn't: it's a severe disability and it covers every single aspect of her life – and ours. Thank goodness we didn't realise that at the time.

Rett syndrome affects girls. They develop normally until they are about 18 months old and then they start to regress, to lose the skills they've learnt, to slip away. Mobility, hand movements and digestion are all affected. Some girls start to have fits. Some are confined to wheelchairs. Insomnia is common. They can live to 40 or die suddenly as a child. The only certainty with Rett is that this moment – now – is probably the best it ever gets.

In the early days after her diagnosis, I'd be pushing her up the high street in her buggy and she'd be screaming and people would stare or say things like: "How can you let your child behave like that?" I struggled for a long time with how to cope with that. It would eat up at me and I'd walk away upset. If I tried to explain Rett to them, they wouldn't know what it was. So I made little cards that briefly explained it. I would hand them out and walk away.

You get these little peaks and dips in her development. At her maximum, she had maybe 20 words and she went through this stage, shortly after diagnosis, where she suddenly started to say new words. One day she said: "Garden" really clearly and we went: "Ah! She's talking!" Then we never heard it again. That makes me think that word is in there and she only managed to express it once. She's got just two words left now: "Mummy" and "Daddy".

Hannah is now six and virtually unrecognisable as the child we once had. She is overweight because she hardly moves and we are struggling to find nappies that fit her – adult ones are too big and children's ones are too small.

She will stay up shouting and bouncing up and down for three hours each night. The only way to help her is to lie next to her and protect ourselves from her pummelling and pretend to be asleep until she settles.

Photos and small things can knock you completely. You never think you'll look back at pictures of your child when they were toddling around at 18 months and find that painful, but the child in those pictures is actually not the child we have now, that child has been taken away from us. If her symptoms had existed from day one, it would still be hard and it would still be cruel, but it would be different somehow.

Hannah is painfully aware of the world around her and her own limitations. The idea that she might know she can no longer turn the pages of a book, that's a terrible thing for her to be aware of. She often gets frustrated and she sinks her nails into your skin and screams – it's her only method of communication.

She is violent to the people she loves – a sign of affection is a bit of a whack, or sometimes she will do her version of a hug, where she falls on top of you. She decides whether she likes you very quickly and if she loves you, she really loves you. There are certain people in the world that make her smile endlessly. She has an instinctive bond with my dad, who had a stroke 22 years ago. It's interesting because he is one of the few people who can't get down on the floor and play with Hannah. He just accepts her as she is.

I know lots of people say you should hold on to today, but we really do. If we put her on her potty after her bath and she manages to wee in it, it is a major house celebration. You just don't take anything for granted. I hear stories every day that terrify me and I have to cling to the idea that we are not going to get to that point.

I heard about a couple whose eight-year-old had never had seizures before and then overnight she was having seven a day. Sometimes that's how girls die. Sudden and unexplained death is a symptom of Rett. When Coleen Rooney's sister Rosie died in January, that was what happened to her.

There are two possible futures for Hannah. One is the worst thing I can imagine happening and one is beyond the best thing – a cure. I don't know how far off that is but I know it's possible in Hannah's lifetime. Girls have two X chromosomes, one is active and one is dormant.

With Rett syndrome, the active X chromosome is the one that's causing Rett. So if they can reactivate the Rett-free silent X chromosome, that will counteract and alleviate the symptoms and possibly reverse the disorder completely. They successfully did it in mice in the labs in 2007 and even the mice who were close to death regained their skills and lived.

Hannah could be as capable as you or me. The potential for everything is there. She has just got to learn it. And she could learn. And that is huge. That's the dream.

When bad things happen in life, you are meant to go through a process of anger, then denial, then acceptance. I think I go through anger and denial on a daily basis, but I will not accept and I don't think I ever should when ultimately, there is still so much hope.

Interview by Emily Jupp

Beth Johnsson writes a blog for The Independent –