The delay
The gap between a new step and a visible result. Most people quit there.
You change something and the outside does not move for a week. That is not proof it failed — it is only the gap. The only question in it: do you stay when nobody is clapping. Almost everyone who quit, quit a little before the thing they wanted.
A question from Seth
What have you been doing every day for three months with nothing to show yet? Do you quit or wait — and until when?