You shouldn't feel obliged to be friends with somebody out of habbit. I know exactly how you feel though, I let my "best friend" bully me throughout school, but as soon as we left and I cut her loose I felt as though I was, well, free. It sounds ridiculous, but I can totally relate to you.
Actually if I were you, I'd probably tell her exactly what I think, as hard as it may be. Yeah, she'll probably hit the roof and hate you for a while, but you really will feel ten times better.
I know it sounds sad, but are you scared of this girl? Are you scared that you'll have no friends if you fall out with her or lose the security of your friendship?
Sometimes friendships that make you unhappy just aren't worth the security they provide, and your friend sounds like a selfish idiot. I'm surprised you've held out this long!