First, learn everything you can about the disorder. Then take care of yourself first. BPD is presently incurable so there is not much you can do besides being careful about your own behavior and avoiding things that trigger BPD. It important to remember that the more central you are to her life, the more she will fear abandonment and the more likely it will be that she will start hating you (devalue you). With a BPD, unconditional love can be counterproductive. Below, i am attaching a summary of what BPD is. I hope it helps to showcase behaviors you should avoid from your end.