My thoughts are with you.
I have personally ‘accompanied’ several young women, cutting themselves.
Without going too much into a hypothesis, I would like to offer you a website, which I have just come across myself.
Following a very interesting concept
Cutting – Self-Injury From the TEENS’s PERSPECTIVE
“I don’t think anyone wants to cut when they are being listened to.” S. Hein
I could imagine, that studying this website, could give you on one hand more information, on the other hand, it might give you a unique way of learning to understand and therefore becoming able to help your daughter in the way, she does need and would like to receive help.
By the way, the fact, that she has ‘chosen’ you as the person of trust, speaks very loud about your POSITIVE relationship to her. Sadly, very often, the parents are the ones, which learn last, if ever about it…
Give it a try, have a look at the website. You might find from there further material to go on your ‘not self-chosen’ journey.
I just wish you now all you need and would like to read more about how you two are doing in this challenging situation. Maybe I can give some more assistance with time.
Right now, feel supported!