My Best Friend is now Best Friends With The grl she bitched abut?

No offense but you sound too clingy. I know that she's one of the most important people in your life but relationships need space. At first while she's hanging out with that girl (the one she b****es about) she'll soon realize that being friends with her was a mistake. Right now she either thinks that she owns you and thinks she can boss you around or she thinks your annoying. For now make new friends. There's most likely plenty of people that you can be friends with. You sound like an Okay person. Don't ignore your friend just try to avoid her for a while (at least a week) . After some time she'll start to miss you and start hanging out with you again. I'm pretty sure she'll approach a conversation with you asking about why you haven't talked to her for that week. But right now it's not what you should be concerning about.