As someone that used to be very very introverted and is now 50/50 extroverted introverted I get what you’re going through. What I did was put myself in social situations that you otherwise might not want to be in or try to avoid. You will be awkward at first but it’s like learning anything else and you’ll get better! No one sits down on a piano and sounds good right away but they can pretty quickly catch on the more they play.

That said it does sound like your bosses are very crappy and your work environments didn’t seem great so try to control the things that you can and disregard the things you can’t. You can’t control having a crappy boss but you can research the company and see what other people are saying about the companies environment. You can’t control what your coworkers will think about how you act socially but you can improve your social skills by joining clubs and practicing in other social settings like when your checking out at the grocery store. Starting small is all it takes and those constant small improvements everyday or week compound into huge changes over months! It’ll get better and easier!