My DB coach helped me think a way was to send a letter talking about the love my W has brought to our family (two sons). So rather than talk about my love for her, or what I miss from her, simply a note about the way her love has made the family.
ME:51 W:46 M:25 S:22, S:20 Divorced 16/9/15 BD 10/12 W left 12/12 with OW, affair confirmed Nov/12. Dark since 6/13 I"m in a new relationship since Feb 14.