Getting off Track

Why is is so hard to stay on track or why is it so hard to get back into the swing of things once you fall off.  Even if its only for a day I find myself searching for excuses on why I can't eat right today, or can't make it to the y today, etc.  Its hard for me to fight this battle as I love working out and the new fitness level I have worked hard for but I still do this battle.  Same with eating, I refuse to live on shakes to loose weight.  I want to eat good to feel good and I do for the most part and there is a definite difference when I eat good and not so good in how I feel.  So once again why is it so hard to do something that feels so good in the end?  That's my question of the day.