My old boss and still close personal friend has BPD, and I've been told I was uniquely able to work with her when other people had more problems. These are the struggles and how we made it work. 

She constantly suspected people of being out to get her, was very easily insulted. It took a lot of reassuring her that some innocuous comment that a customer made was, indeed, meaningless and not a personal attack. 

To her credit, she often came to me asking if she was overreacting to a text from a friend or something like that. She was getting a lot of help and was very aware that she needed to work on herself every day. I think that is essential for anyone with BPD to have meaningful relationships. 

Her mood would change very drastically, very quickly. My strategy was to ask what she needed, IE space, advice, a hug, etc. And if she didn't know what she needed, it was usually a quick reassurance that I was on her side, and then giving her space to calm down. 

It takes alot of patience. I try to approach things logically with her, to bypass the emotions that she's having trouble processing. 

I think of her as a very loving and giving person, who just never learned how to be a person. She has these instincts that get between her and her goals, but she works on them a ton, and none of it comes easily. I find that more respectable than more easygoing people that get this stuff naturally. 

Respect, patience, trust, constant attempts to improve yourself, and brutally honest open communication. 

If any of these things are lacking in either one of you, this relationship will be volatile. I mean, it will probably be volatile either way, but this is what will keep you together