Haven't had a chance to read your whole thread yet, as I am work, so I hope I am not repeating what others have already said.
But......just sit down tonight & TELL HIM. Get a babysitter, preferably overnight, so your kids are not home & sit him down & get it all out there. There is no easy way & nothing about it is going to be easy. My concern is that you are going to do too much thinking & talk yourself out of it.
You can read my story in my profile, but I have not been able to tell my husband of my "A" 6 years ago, because "he doesn't want to know". Have tried several times but he is adamant...."I don't want to know".
And let me tell ya.....the guilt, shame, self-hatred, loathing, and every other emotion does not go away. I have to live with my secret every day of my life & I hate myself for it. I wish I could tell him, but my IC had a good point that after his responses, it is now selfish of me to still want him to know. (BUT....she would not say not to tell him if his response was different. She would be, & was, all for honesty & being open, until I told her about his response.)
According to her, I need to just continue doing the work on my own to fix myself & make sure it never happens again. And it will NEVER happen again!!! I do know that but......
The problem with just working on myself is that how do I know when my "work" is done?? How do I know he won't find out in the future & all hell will break loose then?? How could I ever get mad at him if he did the same to me?? How do I look him in the face & not break down into tears for what I have done to us without him even knowing it was done??? How would my kids feel if they knew?? Etc., Etc., Etc., Etc., Etc., Etc. The mind questions never end.
So much time has passed & due to our circumstances at that time, I am pretty sure he knows something happened, but cuts me off EVERY time the convo has come around to that time in our lives.
And it is HELL living with it & keeping it a secret. I feel the lies continue even though I do not lie to him now & didn't then. The secret itself is the lie that won't go away & I hate myself for it.
TELL HIM NOW!!! Just get it over with. It isn't fair to him & the longer you wait, the harder it will be & then you will be living the "lie" I live every day & it sucks big time!!!
I never had the attachment to my AP you have, so it was easy for me to break it off on my own & never look back. I haven't spoke to him since the day I woke up & thought WFT are you doing. It seems you are apparently still attached though, so you have some issues involved I did not that you will have to work on as well.
Also, did it ever occur to you that your AP's marriage wasn't good either because he was having an A & the guilt may have been eating him alive inside?? Hello.....how can a marriage be good in any way when there is a 3rd person involved?? He probably lied to you just like you did to him about your spouses. His BW probably thinks their marriage is fine & life is good, so perhaps that had something to do with his emotional detachment from you.....or from her because of his attachment to you???
I wish you well & hope it goes OK for your BH. Be there for him in whatever way he needs, or doesn't. Just take whatever he throws your way (figuratively speaking...not actually objects)& continue being there for him. This will be a rough ride for you both but possibly show him the way to SI as well so he can also get help from the GREAT people here.
We are here for you, but please do the right thing & tell him. Trust me....it is better than having the punishment of it eating you from the inside out the rest of your life.