What should a young man do if he loves a girl from his relatives, but she does not reciprocate his feelings, and he cannot bear to leave her?

You must fear Allah concerning your attachment to this girl, and strive to marry her through your family and by involving someone who can persuade her. If she refuses, then seek a woman of religion and good character. Perhaps Allah has chosen this for you because it is good for you, in fulfillment of His saying: (And it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know) [Al-Baqarah: 216]. Be content with Allah's decree and destiny, occupy yourself with what benefits you in your religion and worldly life, and resort to Allah to ease your affairs and choose what is best for you.