Well first of all try not to think of it as undoing your hard work. Eating something that's not primal doesn't undo the wonderful primal things you put in your body yesterday, or and hour ago. Enjoy it and don't feel guilty or what's the point of putting it in your mouth yk? Then get right back on track.
As for not letting fatigue make you reach for the sweets, plan ahead. Pack yourself some primal bars, or bag or mixed nuts or even a heartier snack like devilled eggs or a sort of second lunch.
The more I see the less I know for sure.