Dear Dazed. My young adult daughter is Borderline and I see her behaviors in your (soon to be?) ex.
They don't think the way typical people like you and I do. They do engage in dangerous behavior (sex w strangers, substance abuse, self harm [cutting], bulimia), and also lie to, manipulate and steal from their loved ones.
They can also be very charming, funny and loving when in the mood. They are often very artistic. That's how they hook you and keep you trying to bail them out time after time.
My daughter (22 now) had a very nice, stable, hard working, supportive (and damn good looking) bf for 2 yrs, but as she got sicker, she began to resent him for watching over her too much, policing her. She finally broke up with HIM. And I was glad for him she did.
PM me if you want to talk privately about how her dad and I cope.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve.
~Percy Byshe Shelley