The first section of the letter needs to state what you are forgiving. This may make you feel silly, or there may be some resistance. This is because you do not want to face your past. This is the only way you can overcome your past.
It has to come from your heart. Take your time, as this is personal to you. Imagine yourself talking to a friend and offering them advice. Give yourself compassion in your letter like you would your friend.
State clearing that you are sorry, and the reason why you are sorry. Do not be negative about yourself or try to justify your actions. Be honest about how you felt and how this has affected you, (angry, hurt, sad, ashamed).
Remember, this is for you only.
You will feel a shift in your subconscious, with this simple step to forgiving yourself.