First, I suggest that you look back on your life to concrete examples of your forgiving others. Have you had at least one successful attempt in your past? If so, you have shown yourself that you can forgive.
Even if you have never forgiven someone, you can start now with someone who is easier to forgive than your father. Try to recall someone who has hurt you in the past, but who has not hurt you severely. Start the forgiveness process with him or her and keep at it until you have forgiven. Once you succeed with this person, then try another, again who has not hurt you gravely
Once you have successfully practiced forgiveness on these two people, keep in mind the path that you walked and now apply it to your father. The practice may give you the confidence you need.
Learn more at What is Forgiveness?
In my book, The Forgiving Life, I recommend an exercise that I call the Forgiveness Landscape in which you begin to think about all of the people who have ever been unjust to you. You rate what the injustice is and how deeply that injustice hurt you. You then order these people from the least-severe hurt to the most-severe hurt. You start with the least-severe hurt and begin the forgiveness process with that person. Once you finish the forgiveness process with that one person, you move up to the next person, and then the next until you are experienced enough with forgiveness to start forgiving those who have been the most hurtful to you. This exercise may prove worthwhile for your wife. In other words, she does not start with the parents. As she forgives others, who are less hurtful to her, then her psychological defenses toward her parents, in which she may have been denying the degree of hurt, may change so that she sees the deeper hurt that she has. At that point, she may have the strength, the resolve, and the expertise to forgive the parents. At that point, the nightmares may end. I wish both of you the best on this forgiveness journey.
For additional information, see: How to Forgive.
To order Dr. Enright’s book, see: The Forgiving Life.