There was a story where Derpy was a professor and was brilliant but was a postmare at ponyville because she liked that better. Then Twilight finds out about it. It's a really short story.
Am I making this up? Did it exist? I can't find it anywhere

First that comes to mind is called Derpy's Faking It... but that's not quite a match.

I recall reading that as well, but can't remember the title, and I don't appear to have added it to my list.

7794930
Nah, I'm sure it's that one

7794993
It's not. I checked the story Night Spinner said. It's not the one I'm looking for. Sorry if that was rude

7794920
https://fimfetch.net/story/163600/more-than-its-cover ?

7795223
Yes that's it! Thank you!