I assume she hasn't been diagnosed with anything. I would enourage both of you to go to a therapist together--or at least you can start talking with your therapist. It would be tough to always worry that you would do something to set her off.

Off hand, I don't know of any resources for partners of cutters, but there might be something about it, say, in a book about borderline personality disorder......

She definitely needs help. I hope somebody else will have some ideas here, too!