I have a BAD friend problem?

i have this friend..lets call her stacy. okay well stacy and me used to be bff's. now she is a total *****. sometimes she is the sweetest person ever to me, but then i hear her talking crap about me . she made me cry one time, and she didn't even care. she ditched me at disneyland, and is trying to blame it on me. when i deny it, she tells me im trying to act innocent. but the problem is, sometimes she is really nice to me. so i don't really know what to do.