How can a young man rid himself of attachment to a girl he loved but cannot marry, knowing that he has ended the relationship with her Islamically?

It is forbidden to establish a relationship between a man and a non-mahram (definition) woman outside of marriage, due to the prohibitions it causes, such as betrayal, seclusion, touching, and looking, which lead to fornication, and corrupt and heedless the heart. The questioner must be patient, strive against himself, and keep himself busy away from her. Marrying someone else may be a reason to forget her. He should remember the brevity of this world and that forbidden pleasure is fleeting and its consequences are dire in this world and the Hereafter, for he may die in this state or be punished through his family. He must leave her seeking the countenance of Allah, and fill his heart with the love of Allah and fear of His punishment, and reflect on the consequences of his actions, to see the blessings of faith and piety. As Ibn As-Sammak said: "The concern of the wise is salvation and escape, and the concern of the fool is amusement and merriment." A wise person does not sacrifice the everlasting bliss of the Hereafter for a fleeting pleasure.