I tried answering this before and dclicked on the wrong thing. I apologize.
Sex before marriage hurts the marriage. However, several questions need to be answered: was she a Christian when this occurred? Did she tell you or did you find out from another source? Has she repented? Is she a Christian now? You do not want to be unequally yoked.
God forgives us and requires us to be forgiving. She may have lied because she is ashamed.
I would seek God's will through His word, not through your emotions...Emotions deceive us.
I pray that God will give you the guidance you need.
