My friend always hangs out with this girl I don't like but I call BFF. What should I do?

My friend is really nice and I don't want to leave her. But, she won't hang out with me without this girl. The girl is always bossing me around and hurting me. She always lies to me too. I can tell she doesn't like me but, she won't admit it. She always tries to gang up with my friend and go against me.( my friend is kind of a push over) She told my friend to not hang out with me and she listened. I hung out with other girls and she tried to get me back cause my friend wanted to hang with me. She acted nice for a while so she is my lab partner. After we were lab partners she got all mean. I don't know what to do. I need advice. Can you give me some? (thanks if you did) P.S I am in 6th grade. PSS the mean girl was my first friend and now she is rude and bossy