How can I end this "friend"ship?

Okay, friend isn't really the right word. She's (S) is always hitting me and hurting me, and whenever i get mad, she whines and screams at me cuz she doesnt understand y im upset... She's also always telling me i should wear oush up bras, but i kno im not flat or anywhere near it. And she is really small, so she's a little underdeveloped, but she's always talking about how every guy she sees thinks she's so hot. It annoys the hell out of me. And she only talks about herself, as if every thing in the whole frikkin world was put there specifically just for her. I feel like to her, I'm just a prop, cuz she never seems to care about what i think or what i want. And she'll insult me, and then laugh like nothing's wrong. The only real reason im still "friends" with her is cuz we have the same best friend(we'll just call her L), but she's known L longer than i have, so theyr more like sisters. And L broke up with her boyfriend because he called S annoying, so im afraid if i try and drop this girl, L's going to take her side too. What can i do?