Like isn’t this a serious issue? Think about it, 
If you have really BPD then it’s unlikely that you’ll think you were overreacting to whatever your partner did or didn’t do.. because you have BPD! Therefore you will think your partner is a narcissist or something before you think you have trust issues. Or maybe you go to a therapist because your feeling lonely
due to repeated breakups, surely they will diagnose you if you have a disorder? Well actually they only know what you tell them! So if your judgement is distorted then there’s may well be too (because they are usually
only hearing your distorted side of the story). Even if you take BPD out of the equation, this is a problem in all relationships, how do you know if your right or not! (And yes, I do believe that there is a ‘right and wrong’ with regards to conduct in close relationships. ) do you blindly trust instincts knowing full well that no ones instincts are completely devoid of bias, or do you potentially get caught in a genuine narcissistic trap because you never trusted yourself enough? All this is intensified when you have few friends that your open enough with to discuss what happened- and even if you do the events will still be distorted in your recounting. 


TLDR: relationships are fundamentally are two party story, it your story of events and their story of events. Both are fundamentally distorted. Who do you trust?