I think these relationships are very individual and the time line for personal growth is different for each one, that there isn't a "too late". You are here after 6 years and that is where you are at.
My own opinion is that your T was wise to shift to you examining your reasons for staying. While learning about BPD and DBT is good information for you, that will depend entirely on the person with BPD's motivation to do this, if they will. I think you will gain more by focusing on your own reasons and emotional growth.
I also don't consider staying or leaving as being a success or failure. Each decision is a very individual one and each person makes the best decision for their own circumstances. The person who stays isn't superior to the person who leaves or is better at enduring or making it work. Both staying and leaving are complicated and diffucult decisions in their own ways.
That someone stays isn't always due to them being able to make the relationship better or "normal". I do think the tools on this board and personal therapy help take down the drama in the relationship. We can't change another person but when we change our behavior in the dynamic, there is the possiblity of changing that, and in some situations, it's enough to make the situation better. It does not "cure" the pwBPD. Validating is one of these tools, and it can help, but how much can vary.
In assessing my experience with my BPD mother and father, her behavior is easier to understand. She was the one who had BPD. My father, who in ways was the more emotionally stable one, was more complicated to figure out. I had thought of my mother as being the problem, but he too was a part of the dynamic between them. It was a function of them both.
He stayed, but I think the best explanation for why the marriage endured was again- he had his reasons, and those we can't really know from the outside. It didn't change that my mother had BPD. She was not able to have the insight to work with therapy on that. Some others are motivated to do that, and make progress with that and their relationships. Each pwBPD is an individual too.
These relationships are confusing because- they aren't all good and they aren't all bad. They aren't what we think of as "normal". I could say the same for my experiences with my parents growing up.
We all started somewhere with learning to navigate these situation, and boundaries, made errors, learned from them. Yes-keep in mind- even if you don't do this perfectly, none of us do or did and your saying is still true-
You are not the things she says you are. You are not the person she accuses you of being. You am worthy of being loved and respected, and treated with care and empathy.



