Dear First-Born Child, I can't describe how thrilled I was when you were born. I'd had no wish for anything other than a healthy baby but suddenly, overwhelmingly, my baby girl was all I could possibly want. I'd grown up with brothers and was going to love having a little ally.

As you moved from baby to toddler to schoolgirl, I took pride in your individuality, your robustness. If the girls in the playground were unkind, you took off to play with the boys and were frequently the only girl at their birthday parties. Your favourite T-shirt was bright orange and featured Bob the Builder; you refused pinks and purples. Your thick mane of hair was never adorned with clips or bobbles and you showed no vanity. You championed the underdog and stood up to bullies.

At secondary school, life changed. You had your hair cut short before starting and wore boys' uniform trousers. Children from other primaries began to ask if you were a boy or a girl and you agonised over how to answer.

Your social activities shrank as you declined invitations. School work suffered. You felt awkward with your longstanding female friends. PE and changing rooms filled you with horror. On a dog walk one afternoon, you told me you were in the wrong body – you were really a boy. I said I would always love and support you. You were diagnosed with gender dysphoria by Child and Adult Mental Health Services and we began making regular trips to the Tavistock clinic in London.

Perhaps inevitably, school became increasingly difficult for you. By now you were known by a male name and referred to as "he".

Although popular and outgoing, you were always prone to anxiety and this intensified. Some people were inadvertently cruel, others simply curious and constant questions caused distress. The impact of bullying was devastating. You stopped eating properly because you didn't want to put weight on where a girl would; you vomited; you cut your arms; you said you were a "genderless freak". One evening you went missing and a police officer found you sobbing, frozen on a park bench.

I wish I could say we are through the worst but I suspect we are not: we will need stamina. You have had CBT and take medication to control your panic attacks. Through the charity Mermaids you have started to make vital links with other trans teenagers. You are having time out of the school you insist you don't want to leave so that we can build you up. Your return will be to a reduced timetable. Tonight you cried because your body causes you such distress; you can't have the treatment you want for some years and in your early teens that feels like for ever.

It feels as if I am watching you drown. I see the same child I've always loved, but suffering terribly. You need to believe that while I would do anything to ease your pain I would not change you. The person I love hasn't fundamentally altered or been lost – he has actually been discovered. It's important that you know I am still proud of you, more than ever. You are my wonderful first born and I'm enormously proud that you have the courage to be yourself and take on so many obstacles.

I think most people would have given up on school long ago. It might be that the time has come to try something different while you establish yourself – school is only a part of life after all. With help and the love of your family and friends you will survive this difficult time and it will give you the strength and resilience to hold your head up as the beautiful young man you will become.

I realise my love for you is and has always been completely unconditional and non-gender specific.

Your mum x