Do you think that on certain days, you've had to draw on your resources a lot in order to manage interactions with other people (say, pushy salespeople or whiny coworkers)? After a tiring day, or under conditions of stress (hunger or lack of sleep, for example), your brain would definitely be working differently, and not necessarily to your best advantage in the self-control department.

For me, the challenge is sometimes recognizing that I need to actually stop pushing myself.