What is the ruling on a relationship between a young man and a girl older than him that lasted for three years, and is what he did by establishing an illicit relationship with her until she lost her virginity considered permissible or forbidden, especially with the strong opposition of his family and the young man's desire to marry her?

You must know that what you have done is a grave sin and a great transgression. Therefore, you must repent to Allah for this sin and for your relationship with the girl. You must rectify your relationship with Allah, ask Him, and seek His guidance (Istikhara) concerning your marriage to this girl. You must be dutiful to your parents, obey them, and accompany them in the world with kindness. However, their obedience is restricted to what is good and not in disobedience to Allah, something that benefits them and does not harm you. You should look into the girl's condition: has she repented to Allah? Is she religious and of good character? Try to marry her, as it will cover her and alleviate the injustice you inflicted upon her. If you are unable to do so, try to forget her.