I'm in a really bad relationship, but I don't know what to do

At first, we seemed to get along. We had many of the same interests, we could hold a good conversation, we wanted the same things in life. But as of recently, she has become so distant, she lashes out for no reason, and we have less in common (that is to say, almost NOTHING in common). She refuses my opinions and any advice she is given, and only talks about suicide and how bad her life is.

I didn't have a great life, but I'm trying to see through that. I'm starting to think the only reason we got together was that we both wanted someone because we were lonely.

However, if I break up with her she'll cut more, probably go through with suicide. I can barely keep her from doing that now. Even worse, her father was just diagnosed with cancer. She apologized to me for some of what she said, but she went right back into her old ways in a few hours.

I don't know what to do. This was my first relationship that lasted more than a week, and I don't want her blood on my hands. If I stay with her, we'll both just be unhappy. If I break up, she'll go crazy and most likely kill herself and torture her body (again). I need help.