A mistake is one thing, fathering your neighbors child and hiding it from your family for nearly 2 decades is quite another. How would you, or OP for that matter, know if he has cheated again or is currently cheating, how can you trust anything after that? He was pretty good at hiding his “mistake” - which is an actual human being BTW. OP had every right to believe she was in a happy and stable marriage up until she opened that letter, he robbed her of that, and did it in a really cowardly way. She basically had to stumble upon the information. If OP can look at him everyday and not constantly question their entire life together that’s her business and I don’t feel sorry for her, but his irresponsibility to own up to this from the start has also robbed his other daughter of a father for the entirety of her childhood. Now she’s going to find out that her friend’s dad is actually her dad, like cool... I guess they’re all supposed to just have thanksgiving together and act like everything is normal? Honestly, I think if OP isn’t going to leave, burn the letter and keep the secret between the husband and the neighbor. At least until the kids are older and possibly won’t be so damaged by this type of information. And yeah, LOTS of therapy.