I think it's really ironic when I see these memes, because it highlights how our own heads will highlight the things that grate against our own nerves.
I assume everybody that this resonates with resonates with it because they themselves have been told to go against their nature and wanna know why the other side hasn't.
I'm the other end of that spectrum. I promise you, people told me all that. "Mind your peers. Share the space. Stop being...so much. Learn to be comfortable in silences. What can't you shut the fuck up?"
I have autism, so I didn't understand why I was always screwing it up. I also hated hearing, "Be yourself. People will like you for who you are," because I found that when I stopped trying to mask, I'd immediately hear all of the above all over again.
As somebody with social anxiety, it sounds like your MIL also has social anxiety. Hence, the constant need to have to reinforce that other people can stand her. I had to train this out of myself in my late teens/early 20s.
I know it's annoying to deal with, but showing a little compassion and simply affirming that you're exhausted but her relationship with you is secure will go a long way for her. She'll probably get used to you being quiet around her too.