BPD is extremely, extremely difficult to deal with, for the sufferer and those around him/her.
Leaving her now would reinforce some of the paranoia/abandonment fears that come with BPD and probably seem to make her worse. However, if you think that you won't be able to handle being with her and pretty much sacrificing yourself, leaving her now would be less traumatic than staying with her for long enough to break down some of her barriers and then deciding that you can't take it and leaving.
You could ask to come to one of her therapy sessions and have a heart-to-heart with her therapist about what to expect and a more personal evaluation of what you're getting into.
It all sounds really un-romantic, and a lot of people would say 'love will conquer all and make everything ok' but with BPD, it really won't. Its constant, constant work.
In short, it depends on how much you like this girl so far. Leaving now may seem harder, but its better for everyone in the situation now than later, if it has to happen. I'm not trying to scare you away from the situation, or speculate that you're not strong enough to handle it. She doesn't deserve to be with someone who can't support her the way she needs, and you don't deserve to unwittingly commit yourself to a situation that could break you as a person. Its a very difficult situation- as is any when dealing with mental illness.
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