Willpower is a conscious resource. It draws on attention, energy, and self-monitoring, and all three are finite. The pattern you are trying to override is not finite. It runs automatically, underneath awareness, and it does not get tired.
That is why the same resolution fails in the same place every time. You are not weak. You are asking the most expensive system in your mind to fight the cheapest one, and the cheap one always wins the long game.
Lasting change does not come from pushing harder against the pattern. It comes from changing the pattern itself, at the level where it actually lives: the identity underneath the behavior.