Couples Psychotherapy never works to aid relationships with Borderlines. If anyone tells ya it does, they're lying.
The Borderline is a pathological liar. As part of a couple in treatment, the Borderline is wholly dedicated to winning the therapist over, and convincing him/her they're the one to be pitied, and handled with kid gloves.
Borderline clients bend, exaggerate and subvert facts to fit their own narrative, in order to control the therapeutic relationship and be sympathetically regarded as 'The Victim' within their relationship dynamic... and as solid actors, they're extremely convincing!
Within individual treatment, a coupled Borderline ALTERNATES between praising their lover (to the therapist), to devaluating, vilifying and invalidating them in sessions, week to week. When you're involved with a BPD individual, it's always a love/hate relationship, which has NOTHING to do with you!
This is merely fallout from disappointing, hurtful, abandoning wounds left over from infancy and early childhood experiences with their Mother, and it's called, TRANSFERENCE. YOU are simply the less-threatening, more convenient target for their unresolved mommy issues!
Young/inexperienced therapists unfamiliar to the Borderline's black or white thinking, labile moods, cognitive dissonance and manipulation tactics, fall for this common therapy seduction all the time. The therapist is often pathologically codependent, with BPD traits. They cannot acknowledge these features within themselves, so they can't discern them in their clients or patients.
It's not that you can't remain long-term with a Borderline. It's that you cannot anticipate that this will EVER be a harmonious, loving, mutually nourishing relationship experience, and THAT my dears, is a huge compromise to make, if "Happiness" has ever been something to which you aspire.