--Ann, Vancouver, B.C.
It's important to recognize that a deep marital bond is not absolute protection against an affair, and it would be helpful for you to consider that Tom's mistake is not proof that he loved you less than you believed. This does not justify what he did, but it is possible that he loves you deeply and sees and regrets his mistake. What he obviously doesn't see is that he is still accountable.
Affairs occur in more marriages than most couples realize. We ourselves had to work through this problem. Depending on what study you read, in up to 80% of marriages, one or both partners have an affair, yet only half of them confess. Marriages in which the affair is confessed are more likely to break up. It's important to recognize that confessing to an affair is like lobbing a grenade into the heart of your relationship. And if, like Tom, you walk away emotionally after confessing, you leave your partner to deal with the devastation you caused.
For confession to be effective and healing, it must be accompanied by a complete willingness to assume responsibility for the heartache your partner is going to feel for as long as he or she is going to feel it.
First, don't even consider having an affair yourself: We can tell you from 20 years of helping couples that any act of revenge or of "balancing the scales" only makes the problems between two people worse.
Because you directed your question to Beliefnet, we assume you are a woman of faith and so you have available to you the most powerful force for healing--your relationship with God. You may understand, then, that forgiveness is an inner activity that helps release deep unhappiness and sorrow.
God is love and works through our willingness to forgive. But when we are angry, most of us have to practice forgiveness one small step at a time. Your goal is to shift your attention from what Tom did many months ago to what your mind is now doing to you. The health and wholeness of your mind is within your control.
Forgiveness is a gift we give our own mind, not something nice we do for someone else. Forgiveness does not attempt to justify or explain away the cruel acts of another. It simply allows us to let go of a line of thought that tortures us and can do nothing to rectify the past. When you find yourself thinking anything about Tom and this affair, interrupt the thought and hold him in the light of God's love. If this is not possible, see some figure who represents the divine to you (an angel, a saint, Jesus, a prophet), and watch as this Divine One walks into Tom. Or if you prefer, send words of blessing from your heart to Tom's, such as, "God holds you with arms of forgiveness and love."
Notice that an act of forgiveness never requires that you reinterpret what happened. It is simply a direct way of stopping your own mind from assaulting you. If you will do this every time you become conscious of one of these unhappy thoughts, you will not only be able to forgive Tom, but you will be able to look peacefully at the past and experience fully the relationship you are having in the present.