If any of you would care to take notice, he has already said there is no way he's bailing. He didn't ask for advice on how to deal with having a "broken" girlfriend, he asked for advice on what to say to her when she's talking about being a worthless piece of shit. (Viewing herself as such is part of her greater mental issues, but not the entirety thereof--he wants help with an aspect of her illness, not all of it.)

And my advice OP, would be to say something along the lines of "I love you, and I don't love worthless pieces of shit. You're being stupid." (or silly or something, if telling her she's being stupid would make her upset.) Something along the lines of this insinuates that she is insulting you (and your taste in women/her) by calling herself worthless; her desire not to degrade you might help her overcome her desire to degrade herself. It's worked with several of my friends. I had another suggestion, but I am tired and have forgotten it >.< I'll post it when I remember.