understanding and accepting that she is sick, which leads to adjusted expectations as to what she is capable of
This is a big thing.
As I write on here, I realize things that I did not really have fully thought out, but the process of writing to others helps me see things for me. I wrote this week how I have had to learn to adjust my expectations of what a "50/50 relationship means. And how it's not really 50%/50%, but 100%/100%. So, if I start with 200 points of giving and understanding and give all 200, but my husband only started out with 50 points of giving and understanding, but he gives all 50, we both gave 100%. I just had a bigger reservoir to pull from.
The odd thing about BPD is that many of the traits are developmentally normal in younger children, even in some adolescents.
Yes - they are arrested in their development at some level. They can be smart, funny, intelligent, and some can be successful, likable in most cases, they can care, they can show compassion at times, but when faced with certain input, things that trigger self-doubt or shame (and it can be invisible to everyone but them) they don't have the expected adult level coping skills.
BPD is an emotional disability. When you can look at it with compassion, when you can try to see how much turmoil must exist inside them, it really helps get past the ugly things that can be said, the irrational actions. We have more ability than they do to manage ourselves and more ability to make changes to how we interact with them that can change their learned responses.