Invisible,

sounds like you have been with your GF through thick and thin, if its going on for three years. Can't you be confident in her after such a long time together? I have BPD, and my BF left me after 2 years, not wanting to deal with it, but I never ever ever thought of cheating on him. And this has never ever been an issue with us, although, like you and your girl, we were fighting over nothing so many times. See, all people are different and usually people develop BPD rather early, so it doesn't mean that if she was faithful before - after being diagnosed, she will go far and wide changing partners, no no, this does not happen like this. I am really glad though that you are dealing with her problem and are still with her.