There is no on or off. There is no all-in versus all-out.
There is only a skill spectrum. The things we call discipline, willpower, good decision-making, and so on are just skills. We are more practiced; or we are less practiced. No one is born with self-discipline. We either practice the skill of doing what we say, and the skill of listening to ourselves, or we are out of practice.
Skill in the face of challenge works quite simply. When the challenge is greater than the skill, by definition the skill is insufficient to overcome the challenge. There was no “failure.” You didn’t “fall off,” or have a bad day or week. What happened was the only thing that could’ve or should’ve happened: insufficient skill will not magically do more than a greater challenge.
We would never administer a calculus exam for a toddler who hasn’t grasped basic mathematical concepts AND consider that toddler a failure. Why, then, do we criticize ourselves for “failing” to “make the right decision” about exercise or nutrition? The scaffolding doesn’t exist yet. The skill has yet to be built. The test outcome was exactly what it should’ve been. There was no other possibility until more skill is developed. People will discontinue an effort at getting fitter because they keep playing at a low level of skill. You better think twice about quitting and taking that "break." All that happens is you are even less skilled and less practiced the next time around. Plus, you taught yourself that when you endeavor toward a goal, it's an acceptable option to quit before you have built enough skill to actually work toward the goal.
It isn’t that you “made a bad decision” or “had a bad week” and now you have to “start over.” Instead, you played precisely within your skillset. Practice more. As you are more practiced, your skill will be commensurate or supra-commensurate for more challenges, more days, more pressures.
There is no “off the wagon” or “on track.” But there is skill. And there is practice.