Name your motivation. My motivation is the fact that both my parents passed away at 49 years of age. I am 47, they are my motivation to get healthy. If I keep them in mind, I can overcome any obstacle.
When you slip off track, have a card catalog created to get you back on. Use flash cards and write things to do when you go off track. Pick your usual situations that take you off track, and make a couple cards for each it. (ie: Eat too much, have a card say "that meal is over, start again" or