They can change only if they want to. Personality disorders are a lifelong affliction. That said, commitment to therapy, especially DBT, can be extremely beneficial in teaching remedial behavioral strategies.
A kinder way to view some of the disagreeable BPD behaviors is to think that there is a social/emotional learning deficit at the root. DBT can help people with BPD have more choices in how they respond to events, situations, people.
BPD exists on a continuum; the more traits a partner has, the more difficult (or impossible) it is to have a healthy relationship.
I’ve been married to two husbands with BPD. I knew nothing about this disorder when I was married to the first, but his behavior was so extreme (repeated infidelity, substance abuse, financial irresponsibility, domestic violence), that even if I had known the strategies I’ve since learned here, my relationship would have been miserable.
I’m now married to a wonderful guy who has issues with self esteem and bouts of depression. He can flip flop about major decisions, but he is reliable and kind. He’s done some therapy, not DBT, which is unavailable in our area, but it’s been somewhat helpful. Is he cured? No, but for most of the time, he’s pleasant to be around.
What changed for me was realizing how much of a role I played in exacerbating disagreeable behaviors. Learning not to invalidate was a huge step in a positive direction.
https://bpdfamily.com/content/communication-skills-dont-be-invalidating