Few things are more harmful than an unforgiving spirit in our hearts -- both to us and to the person we refuse to forgive. The hurt they caused us may be real, but if we refuse to forgive someone who sincerely seeks our forgiveness, there will always be a barrier between us.
This is one reason why Jesus made a connection between our willingness to forgive others and God's willingness to forgive us. His words about this are often overlooked -- but He made it clear that if we refuse to forgive others, then God won't forgive us. He warned, "If you do not forgive men their sins, your Father will not forgive your sins" (Matthew 6:15). A bitter, unforgiving spirit simply cannot receive God's forgiveness; it doesn't understand the true meaning of mercy and grace.
Pray for your husband, that God will soften his heart and help him realize the damage this is causing to your relationship. Pray too that you will be an example of forgiveness to him. Pray especially that you will do all you can on your part to strengthen your marriage, and make it all that God wants it to be.
Most of all, pray that your husband will realize just how much God loves him, and that he will respond to God's love by giving his life to Christ. Once he realizes what it cost God to forgive him, he'll be willing to let go of the past.