Ah, thank you! That is it. Just as biting and dark as I remember it. Greg Egan also has a number of stories about people who rebelled against a "many worlds" u

Ah, thank you! That is it. Just as biting and dark as I remember it. Greg Egan also has a number of stories about people who rebelled against a "many worlds" universe by porting human cognition to run on "Quantum Single Processors," which would carefully isolate themselves from the universe until they had made a single decision across every possible timeline. This didn't prevent them from being "branched" by outside factors, but it at least gave them an approximation of free will.