What keeps me motivated is small, short term goals. I know I want weigh 140, but starting at 203 and saying OK - I'm going to lose 60+ lbs is overwhelming. I'm currently working on 2 short term goals, losing 5 lbs in November and to weigh 165 by New Year's Eve. The choices I make are with my next goal/deadline in the front of my mind (ie: if I eat this, I'll never make my November goal!) and the long term goal in the back of my mind.
I also really had to psych myself up for Thanksgiving and allow myself to enjoy it, with the expectation that I would be back on track the very next day - that night even! I planned ahead and told myself that one day (or if I were traveling it would probably have been a couple of days) will not undo my progress and change the good habits I've established.
So, like PP said, if you slip at lunch start over at dinner! We're on a journey to be healthy for life. I was not blessed with the genes that the skinny girls, who somehow don't have to work at it, have. I'm someone who will have to work at it for the rest of my life. I'm coming to terms with that.
Keep working at it everyday! And track everything (good & bad!)