From experience of cutting, if you try to get her to stop cutting, she's going to want to rebell harder and do it more.
I'm astonished that you have the strength to keep up with all of this. I too am a teenager, who used to cut, and this always worried my mother as well.
Try talking to her about what's going on in her life. If she doesn't want to talk about it, or to you then don't push the subject on her. It may cause a trigger(urge to cut) and would irritate her.