I can empathise somewhat here as my DD is difficult where my 2 sons are easier to interact here. Whilst my sons have problems that are easily resolved with emotional and practical (not necessarily financial) support, my DD's are always insurmountable and, if I am honest, made worse by her intransigence. As much as I love her it can be very frustrating to try to help her and I also have felt on many occasion that she is ungrateful. However, I have also realised that sometimes she also needs somebody to vent to and, as somebody she trusts the most, I get the verbal beating she'd like to give the rest of the world. It can be very wearing especially when I'm in pain, tired, etc.
I have found a couple of things helpful. I make it a rule to try not to compare her to her brothers. For whatever reason she finds life far more difficult. She cannot help her nature and she doesn't need me to tell her that she isn't easy.
I don't assist her financially. I might turn up with the occasional item I think she might find helpful or take her out for a meal but I will not be party to an excessive life style she cannot afford. Her idea of essentials is very different from mine! This had major repercussions but she has learned from them and for the first time ever this year, she actually got her tax credits return in the post before I had time to remind her.
I avoid arguing with her. If she gets into that circular argument of arguing about problems that CAN'T POSSIBLY be resolved (even though there are some very reasonable solutions) I stop it in its tracks by telling her I will give her some thought and get back to her. I've learned to bite my tongue (it's much shorter now!) and temper disagreements with, "I'd rather not fall out with you," or "Shall we talk about this later?" rather than entering into a shouting match. I've found that taking things down a notch or two keeps her calmer and I get more satisfaction from our transactions.
I found the following book really helpful. I'm not sure my daughter has BPD but this book gave me some useful strategies for dealing with some of the situations I found myself in with my DD.
Oh, and I've learned that no matter how much I try to show her I love her, it is never enough. She doesn't love or like herself much so she can't ever believe I could possibly love her. I can only do what I can comfortably.