hi 42 and

wendydarling is correct that we are a support group for people with a BPD loved one in their life. youre in the right place.
we do have a great deal of information that may help answer your questions. a good place to start is here:
https://bpdfamily.com/pdfs/10_beliefs.pdfto speak to your questions specifically:
Q) I have read that those with BPD will often “box up” all their emotions and memories of a person they believe has treated them badly as a form of mental self defense to protect from the pain,
i think you are talking about compartmentalization. everyone compartmentalizes to greater or lesser degrees, and someone with BPD traits may do it to greater, even extreme degrees.
one thing that i learned early on after my breakup, that i found both comforting and hard to swallow at the same time, was that my ex and i were on different pages toward the end of the relationship. she had mostly grieved the relationship while i hadnt. the ten beliefs in the PDF i shared speak to this.
keep posting and asking questions 42, it really helps. how can we best support you at this time? are you seeing a therapist?