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.
A valid marriage in Islam has certain conditions and pillars that must be met so that it can be valid, among which is 'Eejaab (i.e. the offer from the wife's guardian) and the Qabool (i.e. the acceptance from the husband), the consent of the guardian and the presence of two witnesses. For more benefit, please refer to Fatwa 83629.
Therefore, if what happened was just an approval and promise for the marriage in front of everyone and the recitation of supplication for successful marriage, then this does not make you Islamically a wife of that young man; rather, this is just an engagement.
On the other hand, the fiancé is an alien (a non-Mahram) woman to her fiancée until the marriage contract between him and her is conducted as we clarified in Fatwa 86143.
Thus, you should know that enabling him to be in seclusion with you and allowing him to do all those mentioned improper actions with you is an obscene mistake. Hence, you are obliged to take the initiative to repent; for more benefit on the conditions of repentance, please refer to Fatwa 86527.
However, since this young man got married with another woman and does not want to marry you, you should not think about him and you should rather ask Allaah to bless you with a better husband.
Besides, you should conceal your sin and not tell anybody about what happened between you and that man, be him a fiancée or someone else. Even if you are no longer a virgin and your future husband gets to know this, then you may use Tawriyah (i.e. saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand), like telling him that virginity could be lost due to jumping or by menses or by some other reasons.
Finally, if this (what happened between you and your ex-fiancée) was penetration in the vagina, then it is not permissible for you to marry unless you make sure that you are not pregnant by observing three menstruation periods according to the view of some scholars or by observing one menstruation period according to the view of some other scholars.
Allaah Knows best.