Feeling left out is the one of the worse feelings a person can have, but many poeple feel this way. What seems to be the cause of this break-up is the fight you had with your new friend, in which your old friend took her side. What you need to do first, is talk to the new girl for whatever the fight was about, try working it out with her. She might be hanging out with your old friend more, because she is mad at you because of the fight. Try to think about the fight you had with her, did you do something that might have annoyed her? Because if your old friend took her side, it has to have been for a reason. Try talking to your new friend and try to work out the fight you had with her, let her say her side of the story, listen to it carefully and consider it. Then tell her how you are feeling about the whole fight, and tell her your side of the story, but whatever you do DO NOT let your feelings explode. And remember to be honest, you might be right in some things but you could also be wrong in other things. And always rememer that it doesnt matter who started the fight, what matters is your friendship.
And always remember, everyone is allowed to have more than one best friend.