looking for expression/idiom that can be used to tell someone to stop thinking they’re more important than they actually are?

Question

Does “get off your high horse” work? What are some other idioms?

Answer 1

the high horse phrase means precisely what you want it to mean, so that is one option.    Lots of others you can use, and even sort of invent, but they all tend to turn on the idea of that person is thinking themselves as being "above" so they need to come down here with the rest of us where they actually belong.

Answer 2

"Get over yourself."