I don't think you need to apologize for not being who he wants you to be.
How about: "I've realized that we aren't a good fit in terms of what we would like from each other. This R doesn't work for me and continued contact with you as a lover or a friend is not healthy for me. I apologize for my poor behavior at some points in our R. I wish you well in the future. So, I am choosing to say goodbye. I'm sure to miss you, but please respect my decision to end this R. No more explanations, no more arguments, no more criticism or demands from either side -- there is nothing to work out because our R is over."
