I think I read this story back in the 70s. The protagonist is consulting with a doctor/psychologist, and believes that his recurring dream of sitting at a window with dead flies on the windowsill may actually be the real waking world, and the world in which this consultation is taking place is actually the dream.
The Cure by Henry Kuttner and C. L. Moore. It was published in 1946 and has been anthologised many times since then, so you could have read it in the 1970s. Possible in A Science Fiction Argosy, which was published in 1972.
The protagonist is Fred Dawson, who is a lawyer. He goes to see a psychiatrist and the scene describing their first meeting is:
They talked over onion soup and through the entree. Hendricks listened, mostly. Sometimes he watched Dawson, though not pointedly. They were in an isolated booth, and, after coffee had been served, Hendricks lighted a cigarette and blew a smoke ring. “You want a snap diagnosis?” he asked.
“You’re worried about something? Do you know what it is?”
“Certainly I know,” Dawson said. “It’s a sort of daydream. But Carruthers told you that.”
“He said you smelled dead flies.”
Dawson laughed. “On a windowpane. A dusty windowpane. Probably it isn’t that at all. I just got the impression, no more than that. I never see anything. It’s a sort of extension of sensory consciousness.”
“It never occurs in your sleeping dreams?”
“If it does, I don’t remember. It’s always a flash. The worst part is that I know at the time that it’s the windowpane that’s real. Usually it happens when I’m doing some routine stuff. Suddenly I get this flash. It’s instantaneous. I feel, very certainly, that whatever I happen to be doing at the time is a dream. And that really I’m somewhere smelling dead flies on a dusty windowpane.”
At the end of the story we discover:
Dawson is from the distant future when humanity is threatened by a mass psychosis and his consciousness has been sent back in time to occupy a body in the 20th century as a form of therapy.