Hi Cora,
I just wanted to join suzn in welcoming you.

Having a relative with BPD can be frustrating at times. I can imagine that this is very hard on you and your fiance.
suzn has given you some great resources--I use SET all the time (and not just with my mother, who has BPD). It's a great tool. Seeing a therapist is also a good thing to do for yourself, as talking with someone who understands BPD can give you some insight and validation.
It's great that you're there for your fiance--I'm sure that it helps him to have someone who loves him and cares about him for support.
What have your fiancee's parents said anything to you about the situation? Are there some limits you could set when she's visiting?
Welcome again, and feel free to join the conversation here!
GG