One reason you may have difficulty forgiving is your pride or because you are still too angry. Or you don't want to be seen as weak. Perhaps you want your spouse to suffer. Additionally, forgiveness can be hard if you think that your mate is not having to pay any consequences for the hurtful behavior.
Kathleen Griffin: "Accepting that what happened to you really did happen is the first step. But it is the hardest step of all."
Source: Kathleen Griffin. The Forgiveness Formula: How to Let Go of Your Pain and Move On with Life. 2004. pg. 5.
What Forgiveness Doesn't Mean
Forgiveness doesn't mean you have to forget what your spouse did that hurt you. It doesn't mean that you have to trust your spouse or reconcile. Forgiveness doesn't mean that your partner won't have some consequences to pay for the pain caused. Forgiveness doesn't let others off the hook. Forgiveness is not a sign of weakness.
Trixie Buckel: "Forgiving is the most difficult thing you ever have to do ... make sure you do it. Forgiving is like putting the cat out at night. Make sure you do it or you will be sorry in the morning. If you can't forgive, it eats away your guts. You can literally get an ulcer. Anger is one of the most useless emotions."
Source: Anne Naylor. "Trixie's 15 Tips For Living to 92." Huffingtonpost.com. 12/12/2009.
Tony Robbins: "The more we judge other people and don't forgive them, the more we can't forgive ourselves."
Source: "Tony Robbins on Love and Self-Awareness (Video)" HuffingtonPost.com. 10/17/2012.
Nancy Schimelpfening, About.com's Guide to Depression, has a definition of forgiveness that is quite doable and an excellent exercise to help you let go of pain.