I admire your forthrightness. There are two camps on this forum -- the non-confronters, and the confronters. Me, I'm a confronter. Worked for me. My husband's girlfriend dropped him like a ton of bricks when my 24 year old son called her and told her to stay away from his father. He was angry at us for a while, but eventually he admitted that he was wrong to have the affair. And then we went to a Retrouvaille marital weekend and started working on our marriage. Their website is http://www.helpourmarriage.org. If you can get your wife to go to a weekend with you, it's an incredible experience.
In the short run, I would buckle my seatbelt, this flight is about to get really rocky. Your wife is going to be very angry about your letter. But as your FIL told you, you were about to be divorced anyway. At least now you can say you did something. Personally, I think it was a very nice letter. Good luck with it.
