Disappointed in myself

For the past week, I have been proud of myself - I have stuck to eating well. I have had more veggies a day, I have not eaten the bad foods (considered bad no matter what diet you are on). I have felt better, slept better, gotten up before the alarm clock and felt great through out the day (though I have been wanting a nap about half way through the day)
Last night I did something stupid. After eating healthy all day, and feeling good - I broke down and had some of those bad foods, that I mentioned earlier, as a midnight snack. This morning I woke up later then my alarm clock would have been set for, I feel like I'm dragging, I have no motivatirn9877777777777777777778 (sorry cat came by) motivation to get anything done today, I only want to sleep. All in all, I feel like I cheated on myself, now I'm depressed. Oh, and I gained weight.
The only thing to do is to suck it up and continue on. As I told my coworker (who is quitting smoking), there are bumps just don't let them turn into mountains. I need to get back on that horse. I need to figure out why I did what I did, and fix it so that it doesn't happen again.