If you think she isn't being a great friend, see whats up. Has she changed because she is going through something? Or is she changing herself for someone/something?
Try to see whats happening and give it time. After a couple weeks, if she is still acting the same way, just slowly disconnect yourself from her. No, this doesn't mean just blowing her off, it means slowly stop hanging out with her.
Sometimes girls can get that way. They may act self centered one week, and be perfect the next. It might be just a phase. Think of all the great times you have had with her. If you still think she isn't the best friend you thought she was, it may be time to find a better friend.