Hi historied, welcome to the group

this is a good spot to land to hear about how other parents have coped with having a child wBPD.
Adoption plus PD sounds challenging to deal with, for both of you. Has your D31 worked with much of her adoption history, as far as you know? I think I have read that current adoption agencies are more aware of and supportive of post-adoption MH challenges than they were in the past.
I suspect she knows her condition.
What tipped you off?
I would like to find a way when we are back in contact to talk lovingly to her about what ails her and how treatment might help. Any suggestions on the best approach.
While each pwBPD (person with BPD) is different, and each relationship is different, generally speaking the disorder is most apparent as the relationships are closer. For example, a pwBPD might have generally normal relationships with coworkers, more off/on relationships with friends, and progressing to volatile relationships with family members. This makes it difficult for someone in an emotionally closer relationship with a pwBPD to be heard -- just so much baggage there.
That isn't to say "tell her"/"don't tell her", more to suggest that effective communication with a loved one wBPD can be unintuitive and take skills and tools that need learning and practice. Fortunately, we have a great deal of support in that area -- there's our section of articles on
When a teen or adult child has Borderline Personality Disorder, and also our "Tools" menu up top in the dark green bar. Talking through your hopes and fears related to telling/not telling her could also be helpful, and we're here to listen.
I also need to take care of myself as her anger, splitting and dissociation did me some real damage during her visit. Any suggestions on how to do that? Thanks.
It is painful being in a relationship with a pwBPD; the more support you can have, the better. Have you tried seeing a counselor or therapist at all? Sometimes having a neutral, supportive third party can be a lifesaver.
Another reputable resources is the
National Education Alliance for Borderline Personality Disorder's Family Connections program. It's a free, 8 to 12 week course that helps family members build communication skills, manage family relationships, and practice self care when there's a pwBPD in the family. Take a look, maybe it could be a good fit.
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Is she in contact with any other family right now (your spouse/partner, any siblings, grandparents, etc)?