I am going to say this, I used to cut myself along time ago, because I hurt so much inside the physical pain was the only thing that could take my mind off of it. So I kinda know what you are going through with that.
But you have to realize that it isn't solving anything, while you cut yourself, those emotions are still sitting there, and if you can't talk to her, find some other way to get them out... Write Bad Goth Poetry, Draw some Pictures, Eat Noodles on a Park Bench Naked if it Helps, Providing there are no kids around.(It would at least be good for a laugh and a Ride home if your Stuck in Town, Cops can be Good like that Sometimes)