How can a committed young man, repentant of committing adultery repeatedly with a married woman, rid himself of his sin, as he feels unworthy of his fiancée?

Sins harden the heart and distance a person from their Lord. Therefore, hasten to sincere repentance, embrace the means of righteousness, avoid the paths of disobedience, beware of the footsteps of Satan, increase your istighfar (seeking forgiveness) and good deeds, and keep the company of the righteous. Do not despair of Allah's mercy, for He accepts repentance and forgives sins. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Whoever repents, Allah accepts their repentance. If Satan causes them to stumble, then they must offer sincere repentance, for all the children of Adam are fallible, and the best of those who fall into sin are those who repent. Thank Allah for granting you the ability to repent. Do not wish for death, and do not despair or lose hope, for this is one of Satan's entry points. As for your fiancée, be keen on marrying her as long as she possesses good character and religion, and do not belittle yourself because of your past sins, for repentance has erased them. Forget what has passed and strive to do good in what is to come.