Help me out here: what is borderline personality?

Perhaps this. Will help: bpd involves mood instability, relationship instability, and impulse control problems.
Poor self soothing. Wikapedia list is thus: borderline personality disorder: extreme "black and white" thinking, instability in relationships, self-image, identity and behavior often leading to self-harm and impulsivity. Borderline personality disorder is diagnosed in three times as many females as males.
Borderline. Is a personality disorder characterised by severe mood fluctuations, chaotic unstable interpersonal relationships, disturbed identity & self-image, chronic feelings of emptiness, impulsivity, chronic recurrent suicidal behaviors, transient paranoea or dissociation, & inability to control anger.
Borderline . Intense fear of abandonment, being manipulative, impulsivity, a tendency to think of people and things in all good or all bad terms, self mutilating behaviors, drug and alcohol abuse, sexual impulsivity, a profoundly disturb sense of self, and extreme emotional volatility.