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.
First, you have to repent sincerely to Allaah The Almighty from your previous unlawful relationship with that man. Also, you must intensely ask for the forgiveness of Allaah The Almighty and perform many good deeds, in the hope that Allaah The Almighty might accept your repentance and forgive your sin.
As for the first young man, if you feel that he has sincerely repented to Allaah The Almighty and regrets his sin and wants to marry you, we advise you to accept him and not the other, since this is better in terms of keeping your sin concealed and preserving your honor.
On the other hand, if you do not marry him and decide to accept the other man, you should not tell him about your past or disclose yourself and benefit from the protective concealment that Allaah The Almighty has granted you, for the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever commits any of these sins should benefit from the protective concealment that Allaah The Almighty has granted him.” [Al-Haakim]
Allaah Knows best.