Oh man, Katie, thanks for that story/advice/thoughts on that, btw. It was actually helpful. It got me to see other people's view on things. Thanks to everyone who gave me advice on that problem.
And bbq, it actually sounds like your girlfriend was in my shoes. Yeah, its difficult being with someone when someone who has been in their life longer likes/loves them, too. Especially if they have treated them indecently in the past. It's like on one hand, "you guys are friends and I want you to be happy" but on the other "she cause all this friction between us because she wants you for her own." Its very difficult, and sometimes the easiest thing is to remove yourself from the equation. Your girlfriend felt that her position in your life was threatened. Instead of causing more problems, she chose to changed the one thing she easily could, her being in your life. I'm sure she is upset about it too, but getting over a relationship is easier than staying in a relationship that is filled with drama caused by other people.
Have you tried talking to your friend about it? Have you tried sitting your friend down and letting her know that, while you do still care about her, she is totally fucking things up for you with other girls? I would sit her down and let her know that she is causing a lot of trouble, but let her know that you do still care about you. I mean it is pretty bullshit that she would talk poorly about your girlfriend to you. That sucks. Just let her know that, while she is an important person in your life, your girlfriend is(was) too.
Then if you'd like to be with this girl, talk to her. Maybe you guys can work things out. Let her know that you've talked to the best friend. Tell her the best friend is an important person to you, but that she is too.