I have (had) a friend who exhibits many of the traits in BPD.
With her, she is unstable in her emotions, self-control, and self-image. This makes friendships very difficult for her.

However, she hasn't been diagnosed with BPD, but I've had to deal with her issues nonetheless. It's good that your girlfriend is getting help, and I think there might be ways you can help, too.

For instance, if you're fighting over nothing... Well, it takes two to tango. My immediate reaction to my friend's behavior was "Help her understand where she's wrong. Help her fix it!" But sometimes you have to pick your battles. Have you tried simply not engaging in a fight if it's not worth it?

If your health is at stake, then you probably shouldn't have a sexual relationship with this person.