Toxic friends

Hailey • Blah
You've probably met one. I have one. Things were good in the beginning. But I've become the butt of every joke. If I can't do something for her I'm seen as disloyal, and when I'm struggling to juggle work, school, and a relationship, she views me as a flake who walks out when in a new relationship. I've seen her be almost an entirely different person with other people and wonder, why won't she treat me like that? But what's interesting is she's awful to her fiancé as well. I don't get it. What's wrong with me? 
As a sufferer of anxiety and depression, this is a stressful situation. And because our friendship was good in the beginning and I put up with all of her meanness, I do feel bad for wanting to get away. 
Actually,I'm afraid to. She knows all the dirt on me. She's terrifying sometimes. 
I'm slowly backing away from her (in addition to actually being really, really busy lately), and part of me is sad. But another part of me is mad because I'm getting called all sorts of horrible things and being bullied in vague but specific Facebook statuses.
She's like a friendly malignant tumor, and I want her gone. 
Does anyone have any good advice on how to get out of a toxic friendship  without starting a war?