All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The questioner mentioned that she had committed an immorality belonging to the grave major sins. Being so, it is incumbent upon her to hasten to turn in sincere repentance to Allaah, by giving up that grave major sin, showing regret for committing it, and deciding not to do it again. Then, she should pray more for forgiveness, multiply righteous deeds, and supplicate Allaah Almighty to Help her obey Him. She also has to conceal her previous sins, and not tear the Screen of Allaah, in compliance with the statement of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam: "Whoever is afflicted by any of those vile things, let him conceal it by the Screen of Allaah, the Exalted." [Al-Haakim]
Concerning the upright person whom she knows now, if he is her husband, she will have to obey him in what leads to no disobedience to Allaah Almighty, deal with him kindly, and benefit from his praiseworthy characteristics. If he is her suitor, and intends to marry her, she will have to accept him as a husband, and not let her previous sins prevent her from that, for whoever repents, Allaah Accepts his repentance, and whoever sincerely repents of a sin is like the one who has never committed such a sin.
As for her fear and grief because of her shyness of committing that grave major sin, this is not right. Rather, one's fear and grief should be because of the apprehension of the Punishment of Allaah Almighty for committing something that contradicts His Sharee‘ah.