I'd agree with you on this one - at least from my personal experience. My guess is that because in sharing personal information, which develops personal intimacy and a bond with someone else, that, for a pwBPD will eventually hit a limit. Intimacy is impossible for a pwBPD, at least over the long term.
What holds her back from telling me other things would be that she's careful of how I would react, let's just say that most likely in the past relationships she had, no one stayed with her as long as I did to connect the dots. That's why she puts those before me in a pedestal that despite them not knowing her illness they still loved her, understood her and was patient with her. Even if she had sex and cheated like it was nothing, she told me that her ex-GF then just accepted her and didn't even get mad. I wonder why that was the reaction of her partner before.
So she only experienced the high times with those ex-partners, sure there may be episodes that are out of this world, but then again they didn't experience it for longer periods of time.
She would always tell me how lucky I am because I had her at her best. She was nastier many years back and wouldn't respect her partner even if she sees how uncomfortable or hurt they were for how impulsive she was and just leaves them just like that.