How can a girl tell the young man she loves that she wishes to end their current relationship due to it being "haram" (forbidden) and her desire to wait for him so that their marriage may be halal (permissible) and blessed?

The relationship with the young man must be immediately severed due to the sin it entails, which is feared to remove the blessing from marriage. The girl should turn to Allah and supplicate with humility and a present heart that He facilitate her marriage to this young man if there is good in it for her, and if not, that He compensate her with someone better. She must strive to convince her family to complete the marriage with ease, and if it is not facilitated, there are many other men.