My DS started to self harm a few months ago and I think we are seeing the light at the end of the tunnel. He was diagnosed with ADHD - it had been long suspected but teachers always said was manageable with different classroom strategies. What we didn’t know is that anxiety, depression, SH, etc. are co-morbidities for ADHD in puberty. He also started after being introduced to it by a girl in his class.
We have professional help (outside the UK so works differently) - online clinical psychotherapy, a psychiatrist (to prescribe meds), and an educational psychologist to guide us and the school on ADHD. He has also been put on an SSRI - which has been transformative. Over the summer (which has already started) we have kept him busy reconnecting with the things he loves.
SH is sadly not uncommon among young people, it seems there is an element of contagion. We have worked to find the source of the contagion and blocked all access to her outside school. We tried every way of talking to him (and shouting) and have found the best to be calm, open conversation where we express concern, love, non-judgment. I don’t know if we’ve cracked it completely but we’re in a much better space to a couple of months ago - unrecognisable really.
Like a pp said, I know multiple people who SHed when young and then stopped as they grew and learned new coping mechanisms. I think about SH as a harmful coping mechanism - trying to turn emotional pain into physical, and use that as my guide - you need treat the emotional pain and the rest will follow.
For therapy - my DS refuses at first until I explained exactly what it is (he had an idea from TV about it and misunderstood). I also explained that lots of people get therapy - the tipping point was that a friend who is an ambassador had just told me she was getting therapy and I was able to say - even someone like an ambassador gets therapy (change the job title to one that might resonate with your kid as being someone ‘important’). He has clinical psychotherapy online (it’s not available here in person) - CBT, DBT, and EDMR. Look for someone or a group that can do both CBT and DBT. (His EDMR therapy is for bullying and something else). Someone with experience working with teens and ADHD.
It is really frightening and I am sending you much strength.