I don't have BP and my partner doesn't but I've had a boss who did and friends. . .

I try to remember my friends with BP are individuals like me. By that, I mean I can understand their problems! They're not "freaks" or "different" or need "special" handling, they are individuals with individual responses, ups/downs, and experiences. I cannot understand another, unless, as has been pointed out by both you, secretname (and Welcome to PsychCentral!) and RomanSunburn, communication happens. I try to keep my "half" of the conversation about me, using "I" statements and ask about my friend/partner's experience, in their own words, and make sure I understand what they say by repeating what I hear and adjusting as they help.

The big thing for me, when around someone with a problem that affects me, is to keep myself as centered as possible. That means learning to make and keep good boundaries; letting the other person know where I "am" and what I'm doing so they don't have occasion for feeling surprised or as if I have been "hiding" and I don't have to wonder if maybe I wasn't clear in my communication or otherwise complicate matters. If I know myself and where my center is, then I can be comfortable and, hopefully, help another who isn't feeling so centered.