Is it considered unjust to distance oneself from a girl with whom an unlawful relationship took place, especially when feeling remorse and desiring repentance? What is the correct course of action to avoid falling into such relationships in the future, and how can one's repentance be sincere? Does a sin result from not marrying due to this experience?

A Muslim must strive against his own self to obey Allah and avoid sins, and to stay away from everything that leads to ruin, including blocking the devil's pathways. Lenience in dealing with a non-mahram girl leads to falling into sin. Therefore, you must repent sincerely, stay away from her, and cut off all communication with her. You are not religiously obligated to propose to her or marry her, unless she repents and becomes righteous; then you may marry her. As for fearing marriage due to the apprehension of injustice, that is from the whisperings of Satan, and you should not abandon marriage for that reason. Marriage may be obligatory in some cases to avoid falling into fornication.