What is the ruling on returning to a former love relationship after repentance, knowing that the other party was harmed because of me, and is there a solution to return to him despite the impossibility of marriage currently?

The Sharia is keen on purity and chastity, and it prevents causes of temptation, setting boundaries for a man's interaction with a non-mahram woman. You did well by ending your relationship with the young man. The best guidance for two people who love each other is marriage. If that is not possible now, then each of you should go their own way and strive to forget the other. You must beware of the whispers of Satan, who beautifies remaining in a forbidden relationship. If the young man perishes because of his love for you, he is the one who brought harm upon himself. Allah Almighty says: "And every soul earns not [consequences] except upon itself, and no bearer of burdens will bear the burden of another. Then to your Lord is your return, and He will inform you about that over which you used to differ."