A distracted mind can be exhausted without producing anything difficult.

You can finish a day genuinely tired, having done nothing that required real effort. Switching costs, half-attention, and constant small decisions drain you at the same rate as concentration, but they leave nothing behind. The exhaustion is real. It just is not evidence that anything difficult happened.

At the end of a tiring day, ask what you actually made or resolved. If the answer is thin, the problem is probably fragmentation rather than laziness. Move one hard thing to the front of tomorrow, before the day has a chance to break itself into pieces.