Bad Friends aren't Friends

Have you ever had a friend that is like a bad drug? A person that pulls you in, excludes you from anyone else and uses you when they want.  I have a class A contender. This person is socially retarded, she says mean things to all of my other friends, but I've been her friend for a while now. I have no idea how to handle the situation. She is all nice and my "Best Friend" until her boyfriend or someone better comes along. She always makes up excuse as to why she can't hang out. It's all total bullish*t. When she's acting like my friend, I have some of the best times, but it's not worth losing all of my other friends that actually like me and are not just using me, Ya know? I can't stand the way she treats my good friends. They don't like her. She doesn't like them. It's always one or the other with her. IDK it's almost like she's completely changed as a person, her values went to sh*t and all she talks about is her boyfriend. UN-FREAKIN-BEARABLE.  I would kick her to the curb but she's like my best friend, I'm used to calling her up and telling her all my secrets. Recently she avoids my calls and only gets in contact if she needs something.  Cutting her out would probably be better for me emotionally but who do I call if I have something I need to talk about? I've known her for so long it would be weird not having her as my best friend, but in truth I haven't been her best friend for a long time. I think I know what I have to do....