Send Me No Flowers(1964)

At one of his many visits to his doctor, hypochondriac George Kimball mistakes a dying man's diagnosis for his own and believes he only has about two more weeks to live. Wanting to take care of his wife Judy, he doesn't tell her and tries to find her a new husband. When he finally does tell her, she quickly finds out he's not dying at all (while he doesn't) and she believes it's just a lame excuse to hide an affair, so she decides to leave him.