Writing a letter like you have done is very important in your healing process. I would like to think that your dad would read it, put some real thought into it, and respond in a reasonable manner. But in all liklihood, it isn't going to happen. That is the power of the cult. But what is important, is that you be true to yourself. What you have written comes from your heart and all of us who share the experience of being shunned by our parents understand why writing a letter to them is more important for you than for them. It is a journal of where you are in time and a reminder that you will always love your father for so many of the things that he did to help you ultimately become the person you are today.
For me personally, writing is great therapy. I never throw anything I write away. I go back from time to time in order to evaluate my progress in dealing with something that I ultimately can do nothing about. Writing my book, Growing Up In Mama's Club, a memoir of sixteen years of my life as one of Jehovah's Witnesses (the Club) helped me rid myself of so much anger that I internalized because I was discriminated against because I did not believe like they did. It also helped remind me how important it is to surround myself with people who can accept me for who I am and what I believe, as flawed as it may be at times, and share a love for the life that I have been blessed with now.