In the 1990s, I read a story in Asimov's (almost sure) about a man who could (by an act of will) change the past, resulting in appropriate changes to the present. I don't remember much of the plot; I believe the man was immortal, and very old. Near the climax of the story, the man recalls with guilt the fact the he altered the past to create the current history of Native Americans. (Before his alteration, things were not great, but nowhere near as bad as what occurred in our history) I don't know why he couldn't change things back.
This sounds like one of the later stories in the setting of "Backspace" -- Winter 1977 Asimov's magazine, by F.M. Busby (including "Balancing Act", "Backup System", and "Wrong Number" in the same magazine in 1981).
Sam, who invented the backspacer (and subsequently, between stories, burned it out while preventing WWIII) did so because for himself to alter history, he had to understand the causes of events. He could fix a smog problem by reversing the rotation of the Earth (so the wind blows east to west instead of west to east) but he can't just arbitrarily undo an event without knowing what went into causing it.
The last story I recall in the series had him changing things so humans gestated their young in the male body (like sea horses) instead of the female one, after Pete (the narrator character) mentions that he wishes he could take over his wife's pregnancy (about which she's been complaining). There likely were others after this, and might well be others between that I don't recall; I quit subscribing to Asimov's in the mid-1980s.