Welcome, cgm mom; glad you're here.
The first thing that I thought of when reading your post is -- if you are open to learning new ways to validate your uBPD (undiagnosed BPD) child, and want to be part of the solution of her taking responsibility for regulating her emotions, I wonder if you've checked out Family Connections:
https://www.borderlinepersonalitydisorder.org/family-connections/Here is how they briefly describe what they offer:
The Family Connections™ program includes:
Education on BPD/ED treatments and the latest research findings .
Relationship skills based on Dialectical Behavior Therapy (DBT) to help support your person with BPD/ED with compassion and empathy.
Support from course leaders and other participants who also have a relationship with a person with BPD/ED.
Suggestions for taking care of yourself and managing your stress.
Effective communication techniques to promote problem collaboration and/or solving.
And yes, if you'd like to figure out how your family as a unit can deal with your D's challenging behaviors, yes, this could be a good fit.
It's so true that you can use all the support and help you can get. pwBPD are high-intensity people and can absolutely drain your emotional energy, no matter how much you love them. It can also be exhausting to deal with how sometimes they are pretty capable, yet at other times they absolutely aren't functional. So definitely check out FC, and also please feel free to post here any time you need to. This is a place where you don't have to "explain" why your child "seems OK" but you're a wreck. We get it here, and are glad you're in the group.
Let us know how you're doing;
kells76