Glad to hear you are in a good place
I'm in a relationship with SO, who has a uBPD (diagnosed bipolar) D20 who stays with us for the summer.
I am also divorced from a man who is uBPD (diagnosed bipolar). My son was developing what seemed to be BPD traits, altho as of now (he is 15), there is no diagnosis.
How does your S13's BPD traits present?
I haven't found any specific resources for this, though I read the book Step Coupling and it was helpful. I took the author's advice to heart and then multiplied it due to the additional strains of having a BPD step child in the picture. The author recommends establishing strong boundaries around the newly formed adult/romantic relationship. As you can imagine, this is not easy for a BPD child.
D20 regressed to a toddler when she was with SO, and many of those regressed behaviors strained our relationship.
I am torn between wishing we kept our own homes and being glad we now live together. I admit it is much easier knowing that D20 is here only for the summers, tho when she is here I go into self-care overdrive.