What I feel like my best friend no longer exists? (Reposted with extra details)?

My friend and I have been friends since year 7, and we were good friends untill about a month ago when she started getting really popular. (We were already both popular but then she moved into the mini slags/chavs popular. The whole popular crowd absoloutley loves her and she's getting caught up in it. I tried to be friends with them but they're all shallow sluts who think of nothing but fags. She's got this weird sense of humor now, and she's behaving really horrible to everyone. She's still nice to me, but more kind of like someone you say hello to nice. She used to be so funny and nice, but now she's just a *****, who laughs at really weird stuff. I know I should try and find other friends, but I can't help just staying and bearing with her, hoping the old her will come back. I keep seeing glimpses of the old her in some of our classes, but she's changed so much it's hard to see the old her. Please help, what should I do?