Mine definitely suffered from social anxiety. It was worse in the beginning, but she got better after a few years of stability with me. She grew up in a ghetto part of town, an immigrant community, and didn't trust "white" people, which is odd, because in her job she has to work with a lot of them. Being in a setting she was unsed to (i.e., out of her comfort zone and out of her control), she was very anxious. She accused one friend of mine being fake to her, when my friend was just trying to draw her out and talk to her. Hid inside the house at one party where there were a lot of families and kids. It was embarassing to me.
Another female friend of mine always hated the sense of fakeness she said my uBPDx exhibited. Other women picked up on this more, though not all. My uBPDx felt more comfortable with certain people (as we all do), but the extreme nature of her social anxiety got triggered more than a non would be. Some of my friends liked her a lot, and she them. Others got bad first impressions and never liked her. She picked up on that.
One
I ignored in the beginning was that she said her old bf's friends never liked her. I should have explored that more.
She and my mom never got along as well, and my Ex was pretty mean to my mom sometimes. I'm ashamed I didn't stand up to this more. It was far more than a typical MIL-DIL tension. Both are emotionally unstable, my mom being more so, though my mom doesn't have that intrinsic meanness that my Ex has, especially when triggered.