Being friends with my best friend is getting too much?

My best friend and I are very different. She is classically beautiful, smart and has a very good personality, she is very balanced in most aspects, I mean she isn't perfect obviously, she tends to sleep around and goes through relationships like tissue, and that makes it so other girls initially hate her (But if she pays attention to them, they love her) and she can get pretty bitchy, but I know that's just the side affects of being very popular. I don't really have those qualities, I'm built like a bodyguard and am not very Intelligent (I use to have helpers from the school but I lost my temper with them) and I have a very dull personality, I know this, and I'm fine with it, but other people aren't. I know I don't match her and so does everyone else. I've heard people ask her why she bothers with me and I've heard them talk bad about her simply because she's my friend. Because I've always felt like she doesn't need me, I tried really hard to be there for her, now it's just natural. She calls me whenever she has a problem, to talk, and I go around trying to fix it, or I talk to her parents and tell them whats wrong (When there are family issues). When girls come looking to beat her up (This has happened 6 times) I step in, and fight for her, physically and verbally. After the 4th time I started talking to people so now I know beforehand if any girls are looking for her and I send her away before they show up and deal with them. When she breaks up with a guy, I'm the one who picks up the pieces and makes sure the boy doesn't hold any resentment to her, so now they come to me if they have something wrong, so now I'm dealing with their problems as well and I'm holding onto their secrets as well It's just getting too much. I know I shouldn't take care of everything for her, but I can't help it anymore, I get nervous when I think something might be wrong with her. I feel like I'm holding her back from growing up &That the best thing I can do for her and myself is stop being friends, but I can't stand the idea of her getting hurt because I failed to be there (It has happened before, where a group of girls beat her pretty bad because I froze and didn't do anything.) I just don't know what to do anymore, I deal with her problems and her ex's problems and my own problems &It's just too much now that we're teenagers (16&17).