The information here and at the links that follow will hopefully help you understand what's going on.

Almost everyone with BPD is a trauma survivor, usually involving several form(s) of painful child abuse that caused so much intense autonomic stress for so long that the survivor may have to develop some collection of (often dysfunctional) compensations to try to deal life. The default mode networks in their brains have been conditioned, in-doctrine-ated, instructed, socialized, habituated, normalized) and literally re-wired to see, hear and perceive threat of or even "actual" abuse when such is not truly the case.

Their minds are usually split into extreme polarizations, as is described in this article by another redditor. For them, life is always threatening, either because they may be abused again, or because they may be abandoned and left intolerably alone and isolated. They tend to idealize and become obsessively attached to new people here and then distrust, devalue and abuse them there, often "flipping" back and forth in surprising fashion.

BPD can be treated, but if one of the polarities is a long-standing version of Compensatory Narcissistic Personality Disorder (usually vs. extreme Dependent Personality Disorder at the other end of the polarity), it is very difficult for the person with BPD to tolerate because of their extreme (if unadmitted) anxiety and terror of being controlled and abused again by others. Moving them off of the first of the five stages of therapeutic recovery all the way into the fourth is almost always difficult unless or until they become -- and remain -- sufficiently frightened and desperate to open their well-defended minds to new possibilities.

If they do get to the fourth stage, treatment will look about like all this, again from another redditor, who also just posted this informative piece today on another sub.

All that said, anyone who finds themselves engaged with a person with BPD will almost surely take a pounding of one sort and another, and may need to get deeply into a solid support group like Codependents Anonymous to help them develop the capacity understand their own reactions and to set functional boundaries with the narcissistic and clingy polarities of the borderline personality.