Simply replace the unfilled areas such as "" and "[date]" with your information.
When you confessed your true feeling to me, I was at a loss for words. Now that I have had some time to reflect, you deserve a reply.
I can't believe that someone as special as you would care so deeply for someone like me. To say that I am flattered isn't enough. You are such an important person in my life. Your [individual's trait] helps me to [achievement]. You have a heart capable of great love, but it is not meant for me.
You deserve someone who will love you as deeply as you love them. You should have someone who will hold you and never let you go, someone worthy of you. At least for now, I can't give you that. I want and need you in my life. However, I don't want to fool you, or myself, into believing that it could be more. It just can't and I don't think I could even begin to explain why.
I hope that we can find a way to continue on after this. I will understand if you need some time to yourself. Please, don't walk out of my life.