Does one sin for continuing to feel affection for a girl he does not see after separation?

A man's inclination towards a woman, without actively pursuing the reasons for it, is not sinful, as long as it does not lead to forbidden acts. Indeed, a person is not held accountable for mere self-talk, as the Prophet, peace and blessings be upon him, said: "Allah has forgiven my Ummah for whatever they whisper to themselves, as long as they do not act upon it or speak of it." However, dwelling on such thoughts can lead to undesirable consequences. If the girl is suitable for marriage and you are capable of it, then proceed with it, for it is the best path, as the Prophet, peace and blessings be upon him, said: "We have not seen for two who love each other anything like marriage." Otherwise, turn away from her and occupy yourself with what benefits you in your religion and your worldly life.