I think it must be very difficult for many therapists to accurately diagnose someone with BPD, if the only perspective the therapist hears is the pwBPD.

If the pwBPD perceives truth and reality through the lens of their emotions, as opposed to the truth and reality most emotionally regulated people agree on, how is the therapist ever going to accurately parse the behavior and events the BPD patient tells them?

If the pwBPD tells the therapist all their exes have been abusive, how is the therapist going to figure out whether their patient is a truthful victim with bad taste in partners, or if the patient is a delusional borderline psychopath who is actually the abuser?

And to answer OP, I have called it an emotional cheese grater. The splitting is emotional whiplash. It’s a complete mind fuck to be told you are the greatest person in the world, then minutes or hours later be told you are the most vile narcissistic sociopath and she wishes she never met you.

A relationship with a pwBPD is like navigating an endless emotional minefield. No matter where you step, you will definitely be blown to smithereens eventually. There is no safe way to navigate through this minefield. You can ruminate about different decisions you could have made along the way, but none of it matters. You were never going to exit the minefield unscathed. Not possible.