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.