[Erin]

Look up some psychology books on learning & conditioning and you'll have your answer.

If you don't consistently practice a newly learned skill (or, in this case, healthy eating habits) every single day, your brain goes back to the old way of thinking. If you can get to the point where you've been eating healthy longer than you were eating unhealthy, then it becomes habit. Until then, you have to conciously think about what you're going to put in your body before you do it each and every time you make a food choice. For many people, this is exhausting and they let their brain take over and revert to the unhealthy habits.