How do I gain closure from an ancient relationship?

Before dating my husband of 11 years (together for 16 years) I dated another guy for about 4 weeks. It was a very sexual relationship that ended badly. He broke my heart into a million pieces and I was devastated for a very long time. I eventually realized that someone was truly looking out for me because he was/is not the kind of person that I would want to share my life with. I love my husband deeply and I am thankful for the life that we share every day. My issue is that my ex keeps popping back into my head throughout all of these past years. I have written him letters (without sending them) multiple times. Time will pass and I will forget about him, but then inevitably I will start thinking about him again. I want him to know how happy I am and that he made a huge mistake by tossing me aside. I want him to know how deeply he hurt me and then for him to know that he can never have me. I know that this is unhealthy and selfish, but I can't seem to get the fantasies out of my mind. I have talked with my husband about this thinking that sharing my feelings with him would help, but it really hasn't. I have seriously considered picking up the phone and calling my ex or finally sending one of my many letters. What can I do to gain closure??