Hi OP, so sorry that you are and your daughter are going through this. I am in a similar place. My daughter told me at Christmas that she had been sh for a few months. She has been very stressed around school,and upcoming GCSEs and also has all the usually crap you have as a teenage girl - friend stuff, boy/girl stuff, family stuff, sibling stuff, hormone stuff. It is a hdifficult time to be a girl, I remember it well! School and the pressures there don't help either in my experience.
My school were pretty good with dd. She has spoken to a wellbeing person at school and is being referred to CAMHS. This might be a route your GP suggests? There is some good info about self harm for parents online too.
My daughter finds it hard to talk about, but she does try which is great I do sometimes talk in the car, but I think it is just as hard there as anywhere.. After a particularly strange episode with school, who completely overreacted about her picking at her scabs in class, I gave her a sei stern 'good cop' style chat in which I told her that I loved her and wanted to help, but that she had to try to be more willing to let me in or i would not be able to make sure that she was getting what she needed. At that point I felt that the parenting of my child had been a bit taken over by school and there were a couple of times where I was not informed of things that would impact on her wellbeing. After that conversation I took a bit more control over how things were dealt with and had a meeting with the school to discuss what I thought should happen with regards to communication with me. I also asked what there was in place to support kids with anxiety etc. So that might be a conversation worth having with her school?
There are all kinds of reasons why young people self harm. It seems to be a growing issue. I discovered that around half of my dd's class are doing it or have done it. This seems extreme to me!
The thing I really wanted to say was please try to look after yourself. It brought up all kinds of feelings for me - mainly guilt - but after reading around some of the info for parents on line I began to feel less shell shocked and more able to understand.
I now check in with my dd every couple of days. She doesn't always like it, but I just ask 'how are you feeling?' And 'have you felt like you would like to sh this week?' And she usually gives me a truthful answer. I try to be unflappable. If she has felt it but not done it I say great etc, if she has done it I might say 'have you got everything you need to sort it out?' Meaning first aid stuff, and then I might say is there anything I can help with? She sometimes opens up, but not usually...! I just try to show her and say that I am here for her. She says it is not one thing, but sometimes she gets so overwhelmed and the feelings of stress etc rise up and she just feels compelled to do it. Like taking the lid of a pressure cooker. She is slowing down though and has not made any new cuts for a month, although she does pick at the scabs when stressed (I still count that as sh).
One of the things I did wa limits social media opportunities, ie phone etc which she was surprisingly ok with. Then I tightened up her sleep and bedtime 'routine' ie I encouraged warm baths, camomile tea before bed and made sure she went to bed at 9.30 in the week, to read a book if she wanted or to sleep. I encouraged proper landline phone calls to friends, which she loves, and we chatted about good friends and what the best friendships feel like. I also got her a good multi vit and some vit d. And I hung out with her, watching friends from the beginning (!!) and having tea and biscuits in the evening before she went to bed or bath. I think she feels better, although it takes time for sh to stop. Hopefully if your daughter has only done it a couple of times it won't grow into a compulsion. It sounds like you are doing all the right things. 