You have to stand up for yourself a little bit more, come right back at her and puncture her bullshit balloon.
HER: "Happy people don't have affairs". She says that she was miserable for 20 years and should have left me a long time ago.
YOU: "Decent people don't lie to their spouses and string them along for 20 years about being happy when they are not."
The thing with her AP cannot be called "love" in my opinion until it has stood up to the test of reality, where they both are free to be together, and they actually are together, for at least a year. What they have is a fantasy of how they wish it would be. Not love. This dude didn't want to spend his life with a 44-year-old with an ex-husband and two kids. He loves his girlfriend. He had sex with your wife. He dumped her so fast it made her head spin.
There are posters on here who got divorced and then remarried. It is possible, but I don't see it for your wife. I have met her carbon copies about a dozen times over the past 20 years. I take it that she is very attractive. She has re-discovered that she can be a rock star, as can any pretty woman - she can go out to a bar, dress a certain way, act a certain way, and guys - even much younger guys - will line up hoping to get a piece. I guess she doesn't realize is that all they want is a piece. Some day, she will. She will meet some other guy, that guy will be decent enough, she will settle down, and she will realize - she may never tell you - but she will realize that it's really all the same. You were not the devil who made her miserable. This guy will leave his underwear on the floor, not lower the seat, whatever - he will not be perfect. Neither would have the spinning instructor.
I am always amazed at how juvenile the cheaters are. Your wife is no exception. That list she made about "defining her losses?" Under him, she lists passion and excitement - aren't they basically the same thing? Common interests? How about building a life together, cooperating to raise two great kids, save money, pay the bills, and comfort each other. Yeah, it got routine and you didn't do the little things, you let it get too much like roommates - but so did she. She has a mouth, she could have told you what was up.
You know, I think you realize it, you said it, but you look weak to her, she has lost some respect for the way you just sit back and took her shit without standing up for yourself. She is going to the gym with macho men - not the spinning instructor, specifically, but if you spend any amount of time in a gym, you know what I mean. She is thinking, why would my husband put up with this from me? My AP never would tolerate, that other guy in the gym wouldn't tolerate it, etc., etc., etc. Maybe if these guys were in the same situation they would tolerate it, maybe they don't tolerate it now because they have nothing to lose with your wife. Me, personally, I have no use for a woman who is going to lose respect for me because I love her that much to put up with her crap. You took vows with your wife, you went out into the world every day to earn a living to support your family, to give her the kind of life where she has time during the day to listen to the sweet talk of a spinning instructor while you are on the job busting your ass to pay for her cheating ass.
I'm starting to ramble. But my point is, get moving on with your life. She can chase you later on when and if she ever realizes just what she gave up.