My friend is such a B****(sometimes) What do I do?

Okay look I know I sound harsh but seriously if you met her and REALLY got to know her I think you'd feel the same way. I feel like she's such a fake, I mean me and my other friend and her go way way back to like elementary school. She's aways had a temper and all, but seriously she's gotten so mean since school started. With some of our newer friends she acts like this nice girl with an attitude, she never says anything super mean or hurtful to them, but omg if I wasn't so fricking shy I would just punch her in the face for some of the things she says. My other close friend feels this way too. I feel like she just doesn't think before she says something and I wish everyone could just see that side of her. She has such a negative energy and it's gotten to the point where I just don't want to see her anymore. She can be real nice sometimes though and does sometimes apologize for some of the things she says, but I don't think she ever means it, what should I do?