Well I don't have BPD, but I'm 99.9% sure I have this unofficial disorder called Maladaptive Daydreaming Disorder. I don't know if you've heard of it, but from what you're saying, it's quite possible you have it. It causes a person to spend hours playing these elaborate scenes in their head that can compare to a movie. I start to talk to myself a lot especially when I get really into it. I make weird facial expressions alongside this. It gets pretty embarrassing if I'm in public and i lose control, so I think we can relate. I'm not sure if I'm really helping you at all though. Oh! But what I try to do if I'm in public is I bring a couple pieces of hard candy to suck on or bring some gum to chew so my mouth is busy doing something else.
I have someone in my family who does that all the time, unconsciously, and sometimes reverts to her 'movie' while I'm in the middle of a conversation with her. I honestly don't think it's a problem but it does show that maybe you're looking for a way to release all the creativity within yourself. Try writing stories and posting them on wattpad.com . The person I know started doing it after becoming addicted to YouTube and reality TV. You'd be surprised how much teens talk to themselves nowadays. But I really think that what you have isn't negative. It's a talent. I can tell you are enormously creative and have a vast imagination. You truly should start writing down your stories or make a YouTube channel :) Good luck, dear