Forgiveness starts when you can let go of negative emotions tied to a memory or situation. Holding onto negative emotions like anger, hurt, loathing, and resentment is counterproductive. Holding onto negative emotions is like holding on to a lump of hot coal while waiting for the other person to start burning. Harboring such negative emotions will only hurt you in the end. You may never forget the experience that led you to feel betrayed, nor do you condone the other person's actions, but by striving to start letting go of negative emotions you can start healing today.
When someone wrongs you, you should strive to forgive, not to punish. Nothing good comes from choosing to hurt someone so you can get revenge. Vengeance and wrath will only perpetuate negativity.
True forgiveness occurs when, and only when, you let go of the negative emotions and put things into the past. By letting you of the negative emotions you are giving yourself peace. So let go, move on, and get peace. Give yourself the gift of forgiveness.