"But now that I've written that dialogue, I know how to make it better."
"I've had a great brainstorm, but Marge has to be in her 20s and just starting her career, not the 40 year-old director she is in the first couple of chapters."
"I'm going to go back and change the setting to Guam, the whole thing will work better."
Don't go back and revise. Make a note in the margin or at the bottom of the page ("more details of the train ride... drunk teenager enters second stop") and keep going.
Why...