
John G. answered 08/09/19
Specialist in Research Projects, Story Continuity
It's hard to go back to an older piece. The key is honestly discipline. Place yourself in the shoes of an editor, and go in with a fine-tooth comb and write out every single mistake you see and change you would make; don't worry about correcting them just yet. These errors could be macro in scale (this character arc needs a little more fleshing out, this scene needs to be replaced) or small (this sentence is a run-on sentence, I should emphasize this particular item more as it becomes important later). When that's done, start making the "big" changes, and slowly work your way down to the smaller ones.
And then, once you've done that, do it again. And then again. And again.
To an extent, you'll never be completely satisfied; but once you start to feel like you're making changes primarily for the sake of making changes, stop, and then do something else. Maybe try to get it published. Maybe start another story. But don't dwell overly long on it after that point.