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 is His slave and Messenger.
It is a great calamity that sexual relationships [i.e. Zina: adultery or fornication] between men and women has become a normal matter in a Muslim country, as this is a habit of Jahiliyyah (the Pre-Islamic era of ignorance) and it is not at all a habit of Muslims, and the will lead to the most evil consequences. Ibn ‘Umar narrated that the Prophet said: “O Muhaajireen (Companions who migrated to Madeenah), Beware of five things if you are afflicted by them –and I seek refuge with Allaah lest they should befall you: if ever Zina spreads in a community until there is no sense of shame on its occurrence or mentioning it except that plague and diseases that were not known in the time of their predecessors will spread among them…” [Ibn Maajah]
Therefore, all those people are obliged to repent to Allaah; Allaah Says (what means): {And turn to Allaah in repentance, all of you, O believers that you might succeed.}[Quran 24:31]
Indeed, you did well by repenting and regretting what happened and you should be careful of every means that could lead you to commit the same sin again, like mixing with non-Mahram men, being in seclusion with a non-Maharam man and so forth. For more benefit, please refer to Fataawa 86527 and 87903.
We do not know from where your fiancé knows that you had already committed Zina. If you had informed him about it, then you were wrong because it is an obligation on whoever has committed a crime to conceal his sin as we clarified in Fatwa 87700.
As regards hiding from him the fact that you have already lost you virginity, then the matter is either of the two following cases:
1- If he conditions that you are virgin, then you are obliged to inform him that you are not virgin otherwise you are cheating him. Then your husband later has the choice to invalidate the marriage contract with you (and take back the Mahr).
2- If your husband does not condition on you to be virgin, in which case it is not an obligation on you to explain to him and it is better to conceal your sin and he does not have the choice to invalidate the marriage in this case [for this reason].
With regard to carrying out an operation in order to reconstruct your hymen, then it is not permissible as we clarified in Fatwa 81575.
Allaah Knows best.