backstage ass
The person or voice in your life that's like an eternal backseat driver. They're always trying to tell you what to do, how to do it, and what you're doing wrong. Over all they're an asshole but probably a correct asshole.
"Dude, Megan is such a backstage ass. If I want to party the night before the SAT, then I WILL party the night before the SAT. She needs to chill out."