Wyllenn put it perfectly. I too have bad days. In the past, I'd have a bad day, beat myself up over it, and say, "Oh well, it's all ruined now." and just keep having more bad days. Now I know it's ok to slip once in awhile. DH wants pizza? Ok, I'll let myself have 2 slices for dinner tonight. But that doesn't mean I can have Taco Bell for lunch and BK for dinner tomorrow.
So long as the good days outnumber the bad, I continue being proud of myself and my motivation stays high.
And you know, letting yourself plateau for awhile can be a good thing. It lets your body get used to the way things are now. Then after awhile, you ca shock it into weight-loss again by re-cutting your calories and upping the exercise.

It's ok to zig-zag down, as long as the trend continues to go downward.
