You are doing awesome! Give yourself an Atta Girl for maintaining your composure while you sort out your feelings.
I acted impulsively almost every time I was given such a challenge...especially when OW was involved. I gave in to my instincts and I regret it.
Holding your tongue until you are on more solid ground isn't going to change anything. You said yourself that he may have kept the note somewhat accessible in the hopes you would find it. This sorta gets him off the hook in terms of holding onto the burden of this big secret, dontcha think?
Personally, I'm all for giving them a long rope to hang themselves. Allow him to carry this burden. Don't make it any easier by handling the heavy lifting for him.
But, I know how hard it is. And, I'm sorry you have been put in this position. You're doing great.
"You know, it's times like these when I realize what a superhero I am." Tony Stark/Iron Man
“Focus on what you can do, then do it with all your heart.” Lois Wilson