Endurance is confidence spread across time.

Confidence in a single moment is a feeling and it comes and goes. Spread across years it becomes something else: a settled expectation that you will still be here, still working, whatever this particular month is doing. That version does not depend on how today went.

When you feel your confidence drop, check whether it is about the day or about the direction. If the direction still holds, the day can be as bad as it likes. Endurance means letting it.