How can I rid myself of the doubts and delusions that pursue me regarding past actions, and is it permissible for me to conceal myself by lying to my fiancée and her family about these actions?

Allah Almighty made the son of Adam prone to error, and He loves for him to repent. He has opened the door to repentance, accepts those who repent, and transforms their evil deeds into good ones. Therefore, hasten to repent, mend your ways, and have a good opinion of your Lord. Do not dwell on the past, and be keen on what benefits you, seeking help from Allah. Do not say "if only," for it opens the door to Satan's work. Seek refuge in Allah from the whispers of Satan and resort to Him, for He is the strong fortress. Be diligent in performing ruqyah on yourself, fulfilling your obligatory duties, and engaging in dhikr. Seek out righteous companions. You may need to consult trustworthy psychiatrists. If you are afflicted with sins, conceal them and do not tell anyone. If you are asked about your sins, conceal them and use tawriyah (ambiguity). Lying is permissible if there is no other way to avoid disclosing the sin. You were wrong to tell the girl what happened with you.