Sometimes once I give up on that, the story comes quite cheerfully along.
I don't only mean theme here. Plot, or central image, or a character element, or whatever: sometimes I fixate on the wrong thing early on. Hanging on too tight makes it slip through my fingers. Like a Rebel Alliance.
My fiction studio prof in college told us about this shrimp metaphor he'd kept wanting to work into a story, and his wife finally had to tell him enough with the damn shrimps already. For me tonight it wasn't shrimp, it was smallpox. So okay: no smallpox this time around. We can do without the smallpox. We can do without the city and the city elders. That is not where we're going from here.
Sometimes I really do have to get it wrong in order to get it right later.
I just wish I could get it wrong more quickly sometimes, so I could move on to getting it right, which is more fun.