Although I can understand why it feels easier to direct your anger at the guy, the person you need to be talking to is your mother. She has put you in a terribly awkward position by behaving in ways that drew your attention and by leaving a “trail” so she would be found out. It makes me wonder if she is unconsciously hoping someone will figure this out and help her put the brakes on.
I realize it’s hard for a young man to call his mother out on something like this but I think you already know you can’t hold this secret. If you can’t bring yourself to talk to her directly, why not simply hand her a copy of the letter you sent to me? Then tell her how unfair it is to put you in the middle when you love both her and your dad. Be clear that you can’t be asked to keep her secret when doing so would damage your relationship with your father. Don’t threaten to tell your dad everything you know. It’s enough to tell him that he and your mom have some things to talk about. Suggest they get some counseling and then leave it to them. Yes, the relationship between your parents affects you. But how they handle their troubles has to be between them.
No matter how old they are, it’s hard on kids when their parents’ relationship is unstable. It’s especially hard when there has been no reason to think that there is tension between them. I’m sorry you are being pulled through this. Do remember that the situation is probably more complicated than you think and that you don’t have to take sides. You can love them both.
I wish you well.