What should a girl do if she has fallen in love with a young man and become attached to him, wishing for him to be her husband, and after they separated, she prays to God to bring him back to her, and asks Him to grant him to her literally, or should she ask for a sign to reassure her?

It was obligatory upon you to fear Allah and avoid forbidden relationships. If this love is inevitable, then let it be through marriage, as the Prophet, peace and blessings be upon him, said: "There is nothing like marriage for two who love each other."

Now that he has turned away from you, do not pursue him, and remember the words of Allah Almighty: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not."

Your feelings towards him or seeing him in dreams do not necessarily indicate that marrying him would be good for you. It is better for you to supplicate to Allah to grant you a righteous husband, and to strive to forget him. If you are sincere with Allah, He will make it easy for you.