My boyfriend constantly minimizes, obscures, changes his story, etc. I realize this is not the same as when he is raging or abusive but in many ways I find this harder. At least when he is raging I know he is being inappropriate. It is the water torture of his minimizing that makes me feel I am going crazy. I start really questioning my own reality.
I know what you mean about starting to question your own reality. This happened to me this morning:
BPD fiance: Can you get the keys to my apartment from the hook by the door?
Me: There are a few sets of keys here. Which one is it?
Her: The one with the green tag.
Me: There's no keys with a green tag. Do you mean the blue one?
Her: I SAID BLUE!
Me: I think you said green, but it doesn't matter, I found them.
Her (raising her voice in a nasty tone): I absolutely know I said BLUE! Why are you arguing with me?
This type of thing happens quite often. We quite literally cannot even agree on the reality of something that happened just 30 seconds ago because she has changed her story.
Regardless, sorry to hear about your troubles with couple's therapy. Sadly my experiences are similar. My own view is that couple's therapy with a pwBPD only makes things worse -- unless you happen to find a very skilled therapist experienced in dealing with BPD.
In our case, the sessions became very unbalanced, with most of the time taken up by her listing all of my perceived problems. She can be very convincing in this mode. The problem is that traditional MC seems to assume the problem is basically 50/50, and if couples would only learn to communicate better, everything would be fine. This does not work when one of the parties is incapable or unwilling to admit they are part of the problem.