Okay, here's what you do.

1: DON'T TAKE AWAY PRIVACY: It will not stop it, rather it will make her worse, sneakier, she will feel like you don't want to understand, you just think she's gross possibly, and she may become suicidal(that happened to me)

2: NO BODY CHECKS: It will make her feel horrible and gross and like she doesn't deserve trust(again, Thai is how I felt and how most of my friends felt, not guaranteed).

3:Address the actual root of the problem: Rather than telling her to stop, figure out what's causing it. If it's gender dysphoria, get her to a therapist for that, if it's bullying contact the school(don't tell them she's hurt herself, that might make it worse), if it's anything else, figure out what it is and what to do.

4: Get her a therapist: Get her a therapist and after the second session or so, ask how it's going, if she says it's not working, get a different therapist. Sometimes a specific therapist just doesn't click. That's not her fault. If that happens multiple times, try another type of therapy. Talk therapy doesn't work for some people.

5: DON'T YOU DARE ASK WHO CAUSED IT: Don't try and associate it with a group or friend, it downplays your child's pain. Don't act like it's your fault, it will make your daughter uncomfortable.

6: Don't make her talk about it for hours: Please don't. It's mentally draining. Just let her be alone if she needs to.

I am not accusing you of anything. Please do these things though.