This is what I think you should do, although it may not be right for everyone or for your girlfriend.
1. Broach the subject gently. Start off broadly by saying something like, “I want you to know that when I see you sad or hurt, it makes me feel sad and hurt too. I want to be here for you no matter what.”
2. Then tell her, patiently, gently, that if she’s been hurting herself, she should not be ashamed. Let her know that cutting is not shameful and she should not feel guilty for doing it. Cutting is an emotional response to pain or even numbness, and she may have many reasons for doing so.
3. So tell her that if she has hurt herself, she has nothing to feel guilty about. You could tell her that she is still beautiful in your eyes, and that her scars will never diminish your love for her. I want you to make sure that she knows that her worth is not lowered simply because she cuts herself.
4. Don’t make her show you her cuts or scars if she doesn’t want to. Let her know that. Say that she can keep them to herself, but that if she ever needs to show them to you, to let everything go, that you will not act disgusted or ashamed of her.
5. Maybe come up with a plan for her, a kind of placebo that she can go to when she’s feeling depressed instead of cutting herself. It could be something as ordinary as punching a pillow or going for a run with you as fast as she can until her lungs hurt, just to feel something. She can even break plates if she wants to. Tell her that all these things can be a great substitute for cutting, and will not cause her physical pain.
6. You could even try carrying antiseptic and bandages around with you, in case anything happens. Tell her that. Let it be her safety blanket. Make sure she has a suicide hotline number in her wallet or bag, and tell her that if anything goes horribly wrong or if she cuts too deep, she should call an emergency number or a hotline, or you will do so for her.
7. Let her know that you wish she wouldn’t cut, that you want to keep her safe, and possibly suggest someone she can talk to for professional help.
8. Under no circumstances should you tell her not to cut herself. That never, ever helps. You can tell her that you wish she wouldn’t do it, but don’t order her to stop.
9. Be patient and kind, and go slow. Don’t do all of this at once. Just work through each step, no matter how long it takes.
10. And most importantly, let her know that you will be there for her whenever she needs to talk, whatever it’s about, no matter how trivial it may feel to you. Make her feel loved and safe.
Go slow.
P.S. Please, if you would, let me know how this works for you. I want her to be safe and well. And tell her that I’m rooting for her, even if she doesn’t know who I am.