For me, no contact has showed me that, sadly enough, the fear had translated in other areas of my life under the guise of social anxiety. Or... Example : ordered a drink in a coffee shop, there was a discount on a specific one, which I requested, the lady makes me pay and charges me the full price, I pointed out there was a mistake, she raised her voice and started acting out, I got triggered, lowered my voice and paid the full price. Left angry and scared.
My mother trained me to freeze when someone gets agitated, even strangers. I got home and my husband was like : "why would you let someone talk to you like that? She makes coffee for a living, you are a professional. Get a grip!" Sorry I don't mean it in an offensive way by the way, I don't judge people who works in the service industry, this is what he said, though, to make me realize I wasn't as assertive as someone in my field should be, that I had to somehow channel the confidence of my title into all situations. Act like the confident adult I should be. This is also the advice my T gave me. I tried. A lot. But everytime someone would raise their voice at me, or berate me, I'd back down, stressed and anxious. My mind knew what was happening and wanted to say something, but MY BODY was shutting down. It truly felt like the response was from my body, not my mind.
I now believe some people internalize the fear so much as children, they lose their fight response, and later on, it manifests as a big lack of confidence and of assertiveness, resulting in an automatic body freezing response whenever there is a threat.
Crazy thing... Started taking martial arts one month ago... One month... Didn't take very long. Two weeks ago, someone started acting out at the store, and I stayed put and was assertive. I wasn't disrespectful, but I wasn't going to allow someone to be disrespectful toward me either. My voice changed, I felt confident, I felt different. She backed down. I wished her a good day. I didn't end up her scapegoat, I stood up.
Martial arts, for me, is a somatic experience that gives the chance to my body to work under stress and "threat". Some sparring can feel quite real, and I survived them all, and I am less and less fragile, everytime I attend a class. Plus there is a community effect that is quite healing. It also desensitized me to touch quite immensely, and I actually enjoy it now, even from strangers. I don't feel as stressed by it. I used to panic whenever I got a massage, and get real anxious. Well, I went to the physio this morning, and for the first time ever, I didn't mind it at all and felt quite comfortable being touched and massaged. A very new thing for me.
Ultimately, for me anyway, it was a matter of triggering and owning my fight response, of getting it back, not just psychologically, but physically. Would it work in front of my BPD mother? Maybe when I get my blue belt...

Otherwise, I am not sure what could help... Is there a specific area of your life that makes you feel real confident about yourself? T said to channel those confident parts. For me, this didn't work... But it might for you.