I can't stand my pessimistic friend anymore, but she still needs me?

Since we don't live close to each other, we mainly communicate through email and Skype (I don't have a Facebook page anymore). Sometimes we can get to having good, pleasant conversations about various things, but I have also noticed over the years that I've known her that she has a pessimistic side. This has come through in full force lately whenever I try to talk to her. I never pester her for compliments, but she gives them to me anyway...if it belittles her and makes her look inferior, something I definitely don't desire. Also, whenever I try to steer our talks into a positive direction, she never fails to bring them down again, in a big way. I've reached the point to where if this is how she's going to be every time I try to talk to her, then I should just stop keeping in touch. But at the same time, she's VERY dependent on me to keep in contact with her. She has extremely low self-esteem, and she's told me that I'm perhaps the only real friend she's ever had in her life, aside from one other person. All her other friends have either betrayed her or stopped caring about her. I don't want to be one of them, but I can arrange to still see her in person from time to time, but I know I couldn't tell her about her ways personally (knowing her, I'm sure she would break down and start cutting herself again or whatever if I told her she was annoying or unpleasant to be around). What can I do?