What should a young man do who fell in love with a girl online and realized how devout she is, and he is facing difficulty forgetting her?

You must repent and seek forgiveness for the forbidden relationship, stay away from the girl, and propose to another girl who is religious and of good character. To cure the malady of passionate love, fill your heart with faith in Allah, ponder His signs, fear Him, and supplicate to Him to turn away her love from you. Indeed, Ibn al-Qayyim explained that the cure for this ailment lies in recognizing the Oneness of Allah and His signs, occupying the heart with acts of worship, frequently supplicating and imploring Allah to turn away such love, and for the servant to return his heart to Him. There is no more beneficial cure for it than sincerity to Allah, for if the heart performs its actions sincerely for Allah, the love of forms will not be able to take hold of it, as it only takes hold of an empty heart.