Momentum is quite literally everything.
The only way to reach a desired lifestyle, which includes your nutrition, work output, exercise regime, sleep schedule... is by take small steps to get the ball rolling; then with every positive action you take giving that ball another nudge down the hill. As it picks up speed, it almost starts to pull you along because the habits become engrained into you. Then the positive feedback loop starts... You feel better so things get easier, and as things get easier you continue to get and feel better. Momentum can also work in the opposite direction, which is why I always recommend that my clients employ a "never miss twice" mentality. If you fall off the wagon once, that's more than okay; but follow up strong with a big win because missing twice (then three times...) starts to tip momentum in the other direction and a habit can begin to form.