"Am I Insane?" by Guy de Maupassant

Published in 1884.

Well that was cruel.

It is rather shocking how many of these stories are lacking in suitable on-line versions. The best I can do for a link to the story is this monstrosity of a URL:
https://books.google.com/books?id=vHEbAgAAQBAJ&lpg=PT3511&ots=85Ff_J8DuD&dq=%22am%20i%20insane%20or%20jealous%3F%20%20i%20know%20not%20which%2C%20but%20i%20suffer%20horribly%22&pg=PT3511#v=onepage&q&f=false