Evan,

Once those negative beliefs become entrenched, they are very difficult to shift. Your girlfriend will need to accept that she is ill and get some help. This is, in a very fundamental way, not your problem, although you are clearly affected by her illness.

She will need good therapy, and lots of it, as I don't believe that borderline personality tends to clear spontaneously.

As for you, I would recommend (it's only my view) that you don't get drawn any further into her destructive game; it can only end badly, because that is where the illness is taking her.

Sex is loving intimacy, not some kind of test, and her attitude here shows the extent of her ilness. I feel very sorry for her, as she is in crisis, and her coping strategy is not working.

Do let us know how things go,

God luck, M