Ok, so being someone who likes to back up his information with research, I will say that the 21 day rule is not a definitive mark for COMPLETELY forming ALL new habits or breaking old ones. Some may take 2 weeks while other habits may take almost a year. The point is this: after roughly 3 weeks of solid effort to eat well and exercise, your new fitness lifestyle becomes MUCH easier to maintain. Remember that it always takes work beyond 21 days, but your discomfort while doing it becomes much less.
Adapting to a Healthy Lifestyle in 3 Phases
1. Excitement to start (Days 1-5): Near 100% compliance
2. Discomfort in change (Days 6-20): They notice the sacrifices that need to be made and realize it takes work to change.
3. Adaptation to the new plan (Days 21+): Their body adapts and they start to feel like they could live this way forever.
Getting through phase 2 is always the hardest. It is the time when you have to break bad habits and your body is going through a number of physiological adaptations. There will be set backs or “cheats” during this time, but as long as they are not too large or too often, you will eventually get accustomed to this new lifestyle come day 22.
Little cheats happen, big cheats set you back to the start line.
So just focus on getting past the first 21 days of your new (or renewed) fitness plan and from there results will come much easier.