I do sincerely feel bad for the girlfriend in this situation. I am convinced that she has some kind of terrible chemical witchery at work inside her. I think this way because I am a victim of such witchery myself, and I have been known to hurt people who didn't deserve it. The irony is, I was suffering myself while I was dealing out that hurt.
The tough thing is that you, anya1991, must also take care of yourself. You cannot let someone else's darkness swallow you whole. So please be careful in measuring how much abuse you will endure, and if it gets to be too much, set yourself free.
I'm kind of encouraged that things seem to be going better at the moment, but let's see how they'll continue. Remember, the amount of crap your boyfriend condones from your girlfriend, says something about how much he cares about you. I guess he has a thin line to walk, but he needs to not let the girlfriend's angry demands suck up all his attention and love.
No matter what happens, I (and lots of other folks here on Polyamory.com) are pulling for you.
Love means never having to say, "Put down that meat cleaver!"