I'm looking for a fantasy story about a Knight of the Round Table with Impostor Syndrome (they don't call it that). He's scared of being found-out, so he volunteers to do every quest, fight every monster, etc. IIRC, the story ends with Arthur explaining that they're all scared and that the volunteering was why Arthur called him the bravest of Arthur's knights.
I don't know when I read it -- definitely between 1990 and 2015, probably between 2000 and 2010.
I was reading a lot of Arthurian stuff at the time, so this may have been the final chapter of a book, a short story -- or maybe even a comic...