My girlfriend cuts herself?

Were both 18 and have been together for 3 years and today she came over and we just really intimate (we've never had sex before) and I was taking off her pants, thats when I saw cuts on her thighs. I asked her who did it and she said me and started crying. I love my girlfriend and to know this really hurt me. I then let her cry in my arms for an hour, I asked her why she did it and she said because she deserves it for how much times she's hurt people. I told her she doesn't deserve it and that I love her and she can talk to me about anything. I asked her when was the last time she cut and she said it was a month ago and also says that she is trying to quit. I told her she doesn't have to go through this alone and she just kissed me and started crying. I love her and really want to help her but I don't know how to. I mean she talks to me about her feelings but I just wanna be more helpful because I'm in love with her.