Can I save my marriage by stating his threat to me?

I had physical relation with a boy before marriage. My husband has come to know that. Now He is spreading this personal matter to his family and my family. He told me to sign divorce form. If I don't sign he will spread this matter to all over my work place. Can I file a case of mental harment? I am Hindu from India . Now I don't have any relation with that guy. He is telling me that he will take information from my college and work place. We have got married according to the Hindu Marriage Act. He has already taken my sign for 'Annulment of Marriage'. Can I say in court " all allegations are wrong and my sign was taken by threatening me?