Several models, including exercises, have been developed to facilitate forgiving.
Here are two:
Model 1 - Three stages:
Rediscover the humanity of the person who hurt you;
surrender your right to get even; revise your feelings toward the person.
Model 2 - REACH:
Recall the hurt; empathize with the person who hurt you; give an altruistic gift of forgiveness; commit yourself aloud or on paper; hold on to forgiveness.
Sources: 'The Art of Forgiving,' Lewis B. Smedes, Ballantine Books; interview with psychologist Everett Worthington Jr.