A person someone who self-harms is in a dating relationship with is usually not a good person to be getting direct help with self-harm from. Support, yes, big-time. But what you can't do is take the place of a therapist or other helper, because to get good help with this, the people who help us need to have more objectivity than a dating partner can, and we also need to be in a kind of relationship with them that isn't sexual or romantic: that's just way, way, too loaded.
So, if she wants to stop -- does she? -- then what you can do is offer to help her find some qualified help, namely, a good counselor or therapist. You could also help her find some books and other resources around this. This workbook, for example --
Stopping the Pain: A Workbook for Teens Who Cut and Self Injure, by Lawrence Shapiro, PhD -- is a good one, and can be found online easily. There is also a hotline in the US for self-harm you could pass on to her, staffed by people trained to help with this. That's the: Self Harm Hotline: 1-800-DONT CUT (1-800-366-8288).
This site online is a goodie:
http://www.selfinjury.com, and can help with finding a counselor (again, based on what she wants, not what you do, since this is about her). This one --
http://twloha.com/ -- was put together (and has a book, as well) by a young woman who was a cutter herself, so it might be one she finds particularly relevant.
If she is interested in seeking out any of those resources and help, then the next thing you can do for her is to just be supportive of her own pace with all of this. You want to avoid asking her about if and when she's cutting: policing her isn't helpful, that just adds more pressure, and also can make a dating relationship pretty weird per its dynamics. You want to avoid asking why, figuring if she wants to talk about that with you, she'll tell you. You can make clear to her, though, that if she wants to talk about this, you will listen, and will do your best to be a good listener who is supportive, and doesn't push or judge.
Perhaps obviously, you'd probably be helped -- and feel more equipped with all of this -- if you looked at some of those resources yourself.
You also can help her by helping you. In other words, what do YOU need around this? How does this make you feel? Staying in touch with your own feelings and managing them -- which includes figuring out where you need limits with any of this, or boundaries -- helps her, but it also takes care of you, which is just as important here. If we can help you with that -- by listening, talking with you about how you feel about any of this, etc. -- let us know, we're glad to support you in this.
