There is no such thing as nice break up letters because in the first place they are symbols of ending a relationship. However, there are ways to write a break up letter that is not too hurting and heart breaking. Softening the details of the letter can work during break ups.
To my ever dearest Jane,
I know that you know that you are a manipulative person. You have always been the boss in our relationship and I respected that. However, I can no longer take the fact that I am the subordinate entity in our relationship. I always wanted to feel masculine when you are with me but it seems the other way around. You always poke me and make fun with me when you are with your colleagues. I never made any comment about it because I love you so much.
I believe that it would be ideal for me to break up with you after you read this letter. It would be best for me to do this before I end up in depression. I would love to be your boyfriend again if you can change your weird ways. Please remember that you will always be my sugar pie.