What is the religious ruling on a girl who is engaged but had a relationship with another man, and now she wants to repent sincerely and fears Allah's punishment? Should she tell her fiancé and ask for his forgiveness, or not? And how can she convince the other person to leave her alone?

A Muslim woman must turn her heart away from attachment to and mingling with non-mahram men. Severing the relationship between a young man and woman before marriage is obligatory, and repentance from it is necessary. Any relationship outside the framework of marriage is forbidden and is considered a type of zina (fornication/adultery) in a general sense. You must repent to Allah for past sins, overcome your self and whims, and hasten to repent before it is too late, while adhering to obedience and increasing in good deeds. Increase your istighfar (seeking forgiveness), for whoever adheres to istighfar, Allah will grant them relief from every worry and a way out of every difficulty.

Repent to Allah with sincere repentance, cut off every forbidden relationship, and strive in good deeds, for perhaps Allah will forgive you and replace your bad deeds with good ones. Seek help through du'a (supplication) during prayer and after it to fulfill your needs, including achieving a righteous marriage. If possible, ask the suitor to expedite the marriage, as it provides protection from the temptations of forbidden relationships. If he cannot marry quickly, then stay away from meeting and communicating with him. If a woman is afflicted with love for a non-mahram man, it is a dangerous disease that must be treated with repentance and avoidance of the causes of temptation. Cover yourself with Allah's covering and do not tell anyone what happened. Be mindful of Allah's watchfulness and fear Him in the unseen, and increase in dhikr (remembrance of Allah) and seeking refuge from Satan to purify your heart from evil thoughts, and compel yourself towards chastity until Allah enriches you from His bounty.