Hello femom,
Considering that you have been shouldering this on your own (it's a good thing you have a therapist) - give yourself a pat on the back, it's so hard sometimes and so stressful... .
When the situation explodes - have you tried to take a "timeout"? It's basically taking some time away safely, so the situation and emotions can come down to a manageable level, assuring the person w/BPD that you would be back. And it also models healthy behavior to your loved one, and perhaps over time they will learn it too.
Here's a link that describes it well, you might find it helpful to reduce stress:
TOOLS: how to take a time outLet us know what you think, and please ask any questions you might have - we are here to support you... .