Hi! I understand why you came here and asked those who have BPD for some advice. I don't know if I have any, exactly, but I can tell you what's worked for me.

My SO and I have been together for nearly three years.
I get outraged, depressed, paranoid, inconsolable, etc. about all sorts of things. But the way that SO deals with it is what makes all the difference in our relationship.
We talk.
I mean, we talk until we're practically blue in the face. I'm honest about what I'm feeling, even if I don't know why I'm feeling it. I never worry that SO will tell me that I'm "just being crazy" (which may be the worst thing anyone could ever say, if you ask me). And, most importantly, SO validates these feelings, even if he doesn't agree with or understand them. He doesn't try to "talk me down", he just tries to find something he's felt that may be similar, and use it to express that he can see why I am feeling the way that I am.
This validation goes the distance for me. Having my feelings validated makes me feel less guilty and angry at myself, which allows me to (eventually) try to see things from a less extreme perspective. I also have to do a lot of work; it has to happen from both sides.

There are a lot of resources for people who are close to someone with BPD. Try to utilize them - they can help. Support groups, the relationship forum, perhaps a T of your own (my SO had one for a while that really aided him in trying to understand BPD). Good luck! Hope this helped a little