What is the path to pleasing Allah after deflowering an engaged girl and cohabiting with her as a married couple, then breaking off the engagement, given the young man's desire to return to her and his fear that she will remain wronged throughout her life?

The questioner's conception of sin is mistaken. The greatest sin is disobeying Allah's command and prohibition by committing fornication/adultery, which is an abomination and an evil way, and Allah has threatened those who commit it with a double punishment on the Day of Judgment. The questioner's marriage to someone other than the girl is not an injustice. It is obligatory for both the questioner and the girl to sincerely repent of this abomination and to cut off all ties between them. The questioner should hasten to marry to protect his chastity.