My spouse had an affair. What do I do?
This is a devastating event in any marriage and it is completely outside of God's plan for the covenant of marriage. In Matthew 19:5 we read, "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh."
You have two options to consider after discovering an adulterous relationship. The first is to allow anger and bitterness to fester until you make yourself and your spouse miserable. Choosing this option often leads to divorce. The second option is to allow God to heal you from deep within and begin to rebuild trust. If you have never accepted Jesus Christ as your Savior, this is the time to do that. He is waiting for you to turn to Him and allow Him to carry the burden of this horrible event.
If you are already a believer, then you have already experienced the incredible washing of your sins and forgiveness of your transgressions that Christ bestows on every believer. Romans 5.8: "God demonstrates His own love for us in this: While we were still sinners, Christ died for us. "When you received salvation, you came to the Lord with a repentant heart. If your spouse has come to you with that same attitude of repentance, you need to ask the Lord to help you to forgive.
No one can know the intricate details of your situation: the level of repentance on the part of your spouse, the history of this kind of behavior, the actual situation of the adultery. It is vital that you seek the counsel of a pastor or a Christian marriage counselor. Be sure that the person you turn to for counseling is a strong, well-studied Bible believing Christian because counselors who are not Christians would most likely advise you to terminate your marriage. But, the Lord desires restoration if at all possible. And we know that in Him, all things are possible.