You know, I think there are a lot of things being said here that are being said "one way" and being heard "another way" so I want to reiterate and maybe re-explain some things.
#1--I can't speak for him, but I believe when 8years said "Back Off" he didn't mean "please be a doormat and let your cheating spouse treat you any way she feels like" because that's a disaster. But so is chasing someone around like a needy puppy constantly picking open a wound! Constantly clinging and wanting to have "relationship talks" is not going to fix this, nor will it make her want to stay in the marriage.
When we say "back off" what we mean is more like this--put yourself in your wife's shoes. You made a gigantic, huge, costly mistake. You got caught and now feel like a heel. Part of the time you cry because you did such an awful thing and now everyone knows; the rest of the time you scream at people because you are defending yourself and don't want to be reminded all the time! She made the choice--that's true--and it is reasonable for her to experience the consequences. But what will catch her eye and be helpful is two things: 1) Showing some confidence in yourself and 2) Expressing some care for her like a best friend--connecting like that as if you value her (knowing that the connecting as lovers will grow out of that friendship and caring).
On her side, she has all the puzzle pieces and she sees the whole picture--and all at the same time it makes her sick but she also misses it! It made her feel cared for! You, on the other hand, only have a piece here and a piece there, and you can't see the whole picture. You keep struggling to see the whole picture. So share that analogy with her and let her know that now you get it--that giving you more pieces hurts her because she has to look at the picture again. But at the same time, not having the whole picture hurts you. Then propose that you two reach an agreement that would work for both of you: you agree to ask her only ONE question a day, and she agrees to answer that ONE question fully and honestly. Then tomorrow you two could agree on how long to ask the questions (maybe 30 days or 60 days). That way she knows it won't be an ongoing Inquisition or feel like an interrogation, and at the same time you'll feel like she's not avoiding you or being dishonest.
#2--I do realize this is a hard concept to grasp, but when she says she hasn't been happy for 5 years, there are two things happening there all-at-once. There is a kernel of truth in that statement--but it is also likely somewhat re-written so she can justify to herself why she would commit adultery. After all, if someone is in misery and long-suffering "for years" or their spouse was "abusive"...why then it's okay right? So there's truth there but it's like magnified.
I'll give you an example. Your daughter is not quite 6 you say, but she says she's been unhappy for 5 years. So not too long after your daughter was born, she wanted to return to work and you wanted her to be a SAHM. She didn't really want that, but didn't really let you know that she felt like that because what kind of mom doesn't want to stay at home? So she agrees...but her heart is a little resentful. As each day goes by, she feels more and more trapped, more and more cut off, lonely, and frustrated because she "doesn't love her own child like she ought to." Even adding one blade of grass every day will eventually, in five years, become a pile of grass! So she it's not like she was miserable enough to say something, but at the same time it was like slowly boiling under the surface.
THEN your daughter goes to kindergarten and she gets a little PT job! Huzzah! She gets to get out a little and not feel too guilty and she feels a little bit good. And someone at work appreciates her work. A man. Then she notices him and decides she'll work EXTRA HARD for him...and maybe dress up just a little. At the same time, you keep pressuring her to quit her little job and she doesn't really WANT to! So she yells, you two fight...and she's comparing the OM who's complimenting her and telling her how GREAT she is, to you who's fighting with her and telling her to stop doing something that makes her feel good. That doesn't go on very long until she begins to say to herself, "You know...I've been miserable for years! Why am I putting up with this?"
See how there's a kernel of truth in that example? She should have told you the truth and said, "I really do not want to stay home. I feel too cut off and lonely here" but she didn't, did she? And as it grew and boiled under the surface she should have been transparent and respectfully requested some change to meet her needs--right? But at the same time, when she did try, you didn't know and you'd work long hours, leave her home alone with all the housework, not say thank you (after all, if she washes your clothes, she's not your servant!), and yell about it or make her feel guilty (or whatever) ...
AND THAT is the kernel of truth. See it? That is the stuff you need to own and say, "Yep that was me. She needs someone to sit an hour or two a night and talk to her!" and then make the time to do that for her.