Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of partner]
I have enjoyed all of our time together and will always value it. However, I think that we both know we aren't right for each other, and sticking together just to try and make something work is only going to preclude both of us from every finding true happiness and ultimate love.
I hope that you find someone in the future that is wonderful to you, that brings you lighter-than-air feelings, and that you can be with until the end of time.
I know that I am not this for you, and wish you the best in all your future endeavors.