Help. GF told me she struggles with self harm. Is ripping me up inside. What can/should I do?

Background: I'm a 23 y/o male. Had issues with depression/anxiety and minor issues with self harm in high school. All that's left now is anxiety and sporadic mild depression. GF [22] of 4 months told me a while back that she used to struggle with self harm in hs. Recently, she admitted that she still has some issues with it (brought on by massive stress). The more she mentions it, the more it seems like it's still a very real part of her life. She was worried that I would "freak out" when she told me. I assured her that I wouldn't. But it does tear me up inside knowing that she's struggling with these urges and hurting herself is the only stress relief she feels she can turn to. And I can't seem to do a damn thing about it. When she told me, I just asked her whenever she feels the urge, to call me up first and just talk to me no matter what time it is or where I might be at that point (class, work, etc.). She agreed she would, but I have a terrible feeling that she won't, for whatever reason.

I need advice/insight/whatever information I can get. I love her very much and care about her immensely. I can't think of anything I wouldn't do for her, and am just looking for a way to be there for her and be supportive.

What, if anything, can I do for her? How do I talk to her about this and voice my concern without sounding like I'm "freaking out"?