Rather than understand what’s going on, they take the therapy itself as worthless because ultimately the problem can’t be internal to them or their thinking. It has to have an external cause.
That's 100% the whole problem isn't it? If they think they're never at fault or the main contributors to the chaos it's impossible to get them to change their thoughts. My exBPD would never have gone to any kind of therapy because, in her own often-used words, she was doing nothing wrong while the whole world shi**ed on her all the time.
Trying to explain that the entire world can't be against her and she is the common denominator in all the life-long chaos with her family, workplace, friends, previous partners and me was just impossible.
Yet having said all that, some posters here say that their partner has actually acknowledged they have a problem and taken some responsibility. All well and good but of course once the BPD kicks in full flow they're back to their 'never at fault' thinking.
I hope things do improve for you, it's such a powerless feeling when we can't get through to them and make them see.