Personally, I would cut myself some slack in your position. For me, fasting takes a little bit of strategizing. I do sort of plan my day around it so I'm not putting in a situation where I might fail. That said, when the unexpected arises, I go with the flow. When you're tired, your willpower falters. Rather than sticking with a fast and potentially not following through and beating yourself up over it, just be gracious to yourself and schedule another day.