What is complete teshuvah? When you confront the same situation in which you sinned in the past; when you have the potential to commit the same sin and yet you abstain from doing so (not from fear but from strength of character), then you know your teshuvah is complete.

For example: Perhaps you were involved in an illicit sexual relationship which has since ended. At some point in the future you find yourself alone with this same person again. Your feelings and desires have not faded, and yet you refrain from intimacy, this is complete teshuvah....

You might ask: What if such an encounter were to occur after many years when you are incapable of committing the offense because of old age? Even this qualifies as teshuvah, albeit a low level teshuvah. Indeed, even if you sin throughout your life and yet repent sincerely on your deathbed, all your sins are forgiven.... [Maimonides, Mishneh Torah, Hilchot Teshuvah, 2:1]