Endurance is commitment surviving boredom.

Difficulty is not what ends most efforts. Boredom is. People are willing to work hard and much less willing to keep doing something after it has stopped being interesting, which is why endurance is better understood as a tolerance than as a strength.

When boredom arrives, treat it as an expected stage rather than a verdict on your commitment. It usually means you have passed the novelty and reached the part where the actual gains are made.