I can only imagine how frustrating, hurtful and invalidating it must feel to you when that happens.
Some folks have no clue as to they why's and how's a disorder works. Nor do they have a clue as to how to even just validate another's feelings, disordered or not. Personally, I like to take times like those to educate them a little bit, but that's just me and I don't have BPD, but my youngest daughter does. I've learned a lot from her and we've worked on our relationship and it's gotten better over the past 2 years for which I am very grateful.
At least with my daughter, it all comes down to how we communicate with each other. What triggers certain issues and how can I help to reduce the trigger to help her manage her way through it more comfortably while at the same time being mindful of boundaries. Sometimes it's a dance and one is going in a different direction than the other, but we always seem to be able to come back together to better understanding.
It can take a lot of work on both sides of the equation but to me, it's all worth it. I'm really sorry you are struggling with this issue and I hope that it lessens for you soon!