Hi balletomane,
I'm sorry to hear this. It must have really hurt to have someone you love react that way. I have to say that just reading your post evoked some turbulent emotions in me, because the kind of behavior pwBPD exhibited toward you makes me feel very angry.
I think

Vitamin C is spot on. I think sometimes when we see someone struggling, it makes us feel afraid, and we can shut down or lash out, even when we want to do the exact opposite. For me, sometimes I feel anger at people who "act" helpless or seem to be playing the victim (I can't "see" their incapacity, or I believe they are more capable than they are) because I have not been able to acknowledge feelings of helplessness in myself. I rationalize that I "refuse" to be a victim, but I think what is actually happening is that I haven't accepted my own feelings of vulnerability and helplessness in some situations, and that people are doing the best they can.
I don't know if this happens with pwBPD, but I wouldn't be surprised if it did. Also, I think sometimes when pwBPD feels needy or insecure, anyone who appears to trump their need in the moment can be seen as a threat, and something that has to be minimized so that the attention will go back to him/her. At least I experienced something like that with pwBPD; it was like, "I had/have it worse than anyone," and it was an eye-opener.
I totally understand your reluctance to ask for help. That is so hard for me! I hope you will not let that stop you from asking, though. Please do. The worst that can happen is that you will get a "no." And that is more information you can use to evaluate your relationships.
heartandwhole