How can I repent and end my relationship with this young man, knowing that he refuses to do so, and the whispers of Satan are pushing me towards the forbidden?

A woman's conversation with non-mahram men via chat and other means is dangerous, and one must repent from it by abandoning it, regretting it, and resolving never to return to it. Allah accepts repentance and forgives all sins. Establishing prayer and keeping good company help in abandoning evil deeds. Do not accept a husband unless he is of good character and religious, with your father's consent and permission. He should not prevent you from marrying for un-Shar'i reasons if the suitor is of good character and religious and you are pleased with him.