A group of science fiction writers are sitting around taking turn discussing their profession, where they get their inspiration from, various things that have happened to them during their careers.
After a series of reasonably normal anecdotes, one of them tells the others that he woke up one morning in a parallel universe, quite similar to the regular world but with enough differences that he could make a living as a science fiction writer simply by writing about everyday occurrences at home. Meanwhile he searched constantly for a way to return.
The others initially take this in good humour before gradually coming to realise that he's serious, and start pointing out various flaws in his story demonstrating how ridiculous this is. Eventually one of them asks if this true, then how did he ever get back home.
The last line is something like
"I didn't", he said sadly. "I'm still here."
(I can't remember if the names used are real sci-fi writers, and a sort of inside joke, or fictional). I would have read it sometime in the mid-80s, probably in an anthology. I think it was in a local library, so the book may have been a few years older.