There was also this:
GertieMotherwell Tue 10-Apr-18 17:43:44
Maybe, if more people reacted like the OP and I, less women would be in this situation.
Not reacting is letting them think it’s ok, and it’s not.
And:
Perhaps, if they were made aware of the impact of their actions or the heartache they’ve contributed to, they would think twice.
Sounds to me as if you wrote just such a letter. I really, really doubt the OW came to you on her knees begging forgiveness as a result, simply because if she had, I'm sure you'd be telling us.
It sounds to me as though you wrote the letter, fuck all happened as a result, and to this day you misdirect your ire at her in order to stay married. That's all your choice, but I don't see the need for such secrecy. You could simply say you wrote the letter, nothing happened but it made you feel better.
I could be totally wrong, of course, but all I've got is what you've said.