Letters 71.35

That which is short of perfection must necessarily be unsteady, at one time progressing, at another slipping or growing faint; and it will surely slip back unless it keeps struggling ahead; for if a man slackens at all in zeal and faithful application, he must retrograde. No one can resume his progress at the point where he left off.

Seneca, Letters to Lucilius 71.35, translated by Richard Mott Gummere