My dBPDw and I have been seeing a marriage therapist for probably about 3 years now, and it has been very little help, in my opinion. Typically, we see the therapist on Saturday, and spend the session re-fighting the arguments we had during the week, then spending the rest of the weekend fighting about the marriage therapist visit. It has been largely circular and pointless. I believe my wife has done an AMAZING job of convincing the therapist how she is the victim and I am the cause of all our troubles. It has become clear to me also, that the therapist has very little experience with, or knowledge of, BPD. I have asked to try switching and trying another therapist, but my wife quickly shoots that down as me not being committed to working it out with her and she doesn't want to start fresh with someone new.
This is almost exactly what has happened in our situation too. I wish we'd never gone to marriage therapy. He just gets to play victim, plus, I feel our T has a rather male bias. BPDh just wants to play victim, not look at how he contributes, and so things don't get better. If all you want to do it blame, blame, blame, and play victim, you stay stuck. As it is/was, I was doing all the work on ME, and after a while I resented that I was working so hard(frankly on things he wanted me to change, but I feel were fine with me), while he just refused to do any self reflection, or work on things that would help the marriage.
I'd advise anyone dealing with a partner with BPD(or likely any PD), to skip the marriage T, and just go themselves. I thought I'd give MC a try, but am now totally regretting it. It just made me feel worse about our marriage, not better.