In it, Kurt was looking through some of his family's old belongings (he might have been cleaning out the attic) and he finds his mother's old diary. He's really excited..until he starts to read it. There are entries about how she was worried about his girlish behavior and the possibility that he might be gay, and she uses derogatory slurs like f*gg*t. Kurt's heartbroken, of course. I think when he asks Burt about it, Burt says that he lied to Kurt to protect him, or something like that? And there were some hurt/comfort elements. I don't remember if there were any pairings, but if they were, they weren't central to the story.
I really loved it because it was so unexpected and unusual, and the only fic I've ever read with a homophobic Mrs. Hummel.
(Edited to add more details)
FOUND! It's Attic by the always amazing miggy
Thank you to kveoyna, who found the author, and remuslives23, who found the fic!