My ex girlfriend is crazy but I still love her, what should I do?

She suffers from depression and I understand this but it does not give her a reason to use that as an excuse for everything. We broke up at the start of April after a 3 month relationship because she claimed she couldn't cope with a relationship and other feeble excuses, like her depression means she can't love. She has really low self confidence so she would always say that she is crap for me and I could do so much better but I would always disagree. Anyway, she started dating me again on the 15th and everything was going good again, and then we lost our virginity together. Everything was going great, we were happy together and everything. Then on Wednesday she was obviously having a bad day, she broke up with me again, claiming that she can't keep her mind the same and that she doesn't know what falling in love is, but she loves me but she's doesn't know if she is 'in' love with me. I obviously argued with her and tried to make her change her mind. She apologised and said she still loves me and wants to be my best friend still. I said yes because I love her and so everything sort of went ok, she held hands with me briefly on thursday when I saw her at school. Today we went to the cinema and held hands because we were scared of the film and I lent in and kissed her. But we spoke after the film and she said she still didn't want to be with me. I gave her this letter I wrote about how i love her and I can change so that we don't argue. She still said no saying that she would mess it up again. It seems when she is sad she doesn't want to be with me but when she is happy she does. So I asked what happens when you want me back when you're happy again and she said she will ignore it and won't get back with me. She still obviously has feelings for me, but at the same time she can't seem to cope with a relationship. What should I do? I need to make her miss me but I see her at school and stuff. Should I never go back with her? Stay best friends?