What to do when your best friend is being a ***** ?

We've been best friends for years now and we've always been inseparable, but lately she's just been thinking only about herself and is making everyone and everything run around her. She's been making arguments with me because i'm friends with someone that she doesn't like for no real reason and that my closest guy friend calls her stupid for a joke on the rare occasion ( only because she's been pretending to be stupid and asking that most brain-dead questions on earth). We're meant to be going on holiday together next month but at this rate something bad is gong to happen and I don't know what to do?