Endurance compounds skills even while results appear flat.

Skill accumulates on a different schedule from results. You can be visibly stuck for a long time while your actual capability rises steadily underneath, and then the results move suddenly when the accumulated skill crosses some threshold. The flat stretch was never wasted.

During a flat period, track ability rather than outcomes. Are you better at this than a year ago? If yes, keep going. The results are a lagging indicator and they will catch up.