I read this story sometime in the '70s in one of the prozines of the time, but it might have been a reprint or just from an older issue acquired in a used bookstore....
There's been some kind of apocalypse with just a few survivors, who are capable of psychic time travel. They can only visit times from before when each was born, and the time stream keeps compensating for whatever changes they make. Each one is pursuing some kind of pet project - one is trying to keep Lincoln from being assassinated, and another remarks that when he hears people later in the 19th century talking about "Ape Lincoln", he knows that the first has succeeded, but the changes are always overwhelmed.
One of the characters works out a very elaborate scheme requiring all of them making changes that'll reinforce each other enough to make a lasting change, but it doesn't work, either.
Help, please, if you can. :)