Interesting that people are so unanimous and so clear in their minds. I feel a very strong desire to write to the OW in my life.
At the moment, I am thinking that I will write to her in 25 years, when she reaches the age I was when I found out that she had been in my my life for 3 years.
Of course my husband is a great deal more culpable than she is, and I have watched him suffer and fall apart under the weight of guilt and unhappiness.
She, young and unencumbered, with no kids, no bereavements, no deep disappointments, and her whole life in front of her, has blithely moved forward, ready to enjoy the next new experience that life has to offer. I get the feeling that she is a nice person, enthusiastic, positive, cheerful, friendly, easy going. I know that my husband never lied to her about our marriage.
She never promised me anything and does not personally owe me anything more than what we all owe to all our fellow humans (perhaps all our fellow creatures). I think that includes not doing anything for our own whimsical pleasure which clearly and demonstrably causes tremendous pain to someone else. What she did has hurt me and one of my kids more than I can explain. The fact that she never specifically promised not to collude in my betrayal doesn't make it ok to do that.
I want her to have a moment when she realises what it means to watch the family which has been the focus of your life just turn to ashes in front of you. I think when she is in her late 50s she might look back with more understanding, so I will wait for her to grow up a bit, and if I am still alive and still care, I might write to her.