Hi prettypictures and welcome to the boards

What RRRJCCN said makes sense -- it can be so validating when someone else says "Yes, I see it too." I'm glad your sister could be there for you.
The car experience sounds like it cemented for you that your D27 is dealing with issues beyond just "launching" or "adulting". There is something serious at play, and as you reflect back on your family history, pieces are falling into place for you.
Parenting is full of regret, there's no way around that. One thing you'll read here sometimes is "when we know better, we do better". We didn't know back then the best way to love our children, but we can learn new ways and move forward differently.
I think I'm reading you correctly that this:
I don't know what else to do until I can confront her with my suspicions. I don't know how I'm going to say it, but I know someone needs to tell her to get help.
Any advice from other parents dealing with this would be greatly appreciated.
I'm not sure what else I can do to help her, or if I even am helping.
means telling her that you suspect she may have BPD? Did I get that right?
What would be your biggest hope, the most ideal outcome, if you were to have that conversation?