Is she worth my friendship anymore?

[Sarah]

Leave her and find other friends. Sorry, but it seems like she has done more bad for you than good, and the little good things she does could never make up for all the really bad things she does. I know you think she's nice, but if she acts like that than she's truly not. There is plenty of other nice people around, and they're probably all a lot nicer than she'll ever be. Break your somewhat "friendship" before she does something really bad that could hurt you or something like that. I'm sorry, and I know it might be hard, but hang in there. You'll probably have a blast with some new or old friends (maybe both) and realise you don't need trouble and stress like her in your life.
I hope this helps!