How to deal with a "toxic" friend?

I've known this girl for about 8 years, so it's hard for me to just cut her out of my life as some people suggest because although she has a bad side to her I also know her for her good side, so I would not want to lose her as a friend. The problem is she is one of those people who is always acting like a victim and somehow attracts drama and misfortune into her life. I read that those people usually spread that type of bad luck to their friends, and I definitely don't need any of that drama in my life. She is always pessimistic and seems to get some sort of kick out of self loathing and talking about how bad she has it. I don't understand this girl - I try to get her to be more optimistic and she's telling me that I'm just sugar coating reality. Even though some of her problems come from an unhealthy relationship she chooses to stay with the person and continue to whine about how her life sucks.