As everyone has told you, she needs to see a psychiatrist, to see if meds will help her, and she needs to see a therapist regularly, and attend a support group for cutters.
It sounds like her main problem is that she doesn’t believe her parents will help. I’m sure she has good reason to believe this. I never would have shared—well, I never did share my problems with my parents at you age. Or afterwards.
It would help in figuring out a strategy to get her help if we knew the specifics about her relationship with her parents, and why she doesn’t trust them. Clearly they have problems of their own. Maybe they don’t believe in psychology. Maybe they’ve abused her. Almost certainly they’ve beaten down her self image until she thinks she’s a lump of shit.
That doesn’t mean they won’t help her. Maybe if she comes out, and shows them what she is doing, and asks for help, they might all get help. That would be best.
First, though, she should find another adult to talk to. Preferably someone she trusts, but it could be a counselor at school. She needs the help of someone who is trained to deal with these things. I know you love her, but even with all the best advice we can give you, you will not be equipped to help her. Like @blueiiznh said, your relationship with her is too intertwined for you to be able to gain the distance you need to help her.
She *has to ask an adult for help!!!!!*
The issue then becomes who? If you tell us about the adults in her life who aren’t her parents, we can help you help her strategize about who might be best to talk to. She should tell them what she’s doing and what is wrong with her relationships with her parents, and ask how she can get help without making things worse with her parents.
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This is for your understanding. As @mrentropy said, she is doing this as a coping mechanism for a deep psychological pain she feels. When you are depressed enough to want to die, you have to distract yourself. Cutting yourself causes pain and it helps you focus away from what is really hurting you. I’m sure she wants to die because her relationship with her parents makes her feel like she’d be doing them a favor by dying. Something along those lines, anyway.
She may not be conscious of her motivation. That’s why therapy is so important. It will help her see where her pain comes from. It will also help her figure out how to cope with it.
Oh. Another idea. What is your relationship with your parents? Another way to take this is to get them involved, assuming they are better parents than hers. We had a question from CAK a while back about whether her daughter should inform her friends parents that her friend was pregnant. The friend was deathly afraid of them. Something like that, anyway.
The consensus was that the girl needed help. Your love needs help. You may feel it is being disloyal to her to tell your parents, but don’t forget, we are talking about a life here. Your parents may not be the right ones, but you’ve got to find someone you trust.
@Vincent_Lloyd I do believe you are a smart person. I’m sure you want to save her, yourself. I think you realize you are out of your depth here. I applaud your impulse to help and your realization that you are not someone who can provide the help she needs. I wish you luck in this. Please get an adult, preferably an expert, involved.