I feel the need to say that even if this is a mid-life crisis, his behavior is NOT a normal male stage. None of it is normal, whether male or female.
With that said, you cannot control or predict what he is going to do, but you CAN take control of your life. When you take control and move your focus away from him and back on yourself, the rollercoaster WILL improve.
Big hugs. Hang in there.
"Sometimes it takes a good fall to know where you really stand."
This is not normal. Well, maybe feeling a bit out of sorts when you hit middle age is, fearing your mortality, wondering about the mistakes you have made in your life, and whether you made the right decisions with your life choices - that might be normal. What is not normal is choosing to have an affair with someone instead of working through it with the help of a the love and support of the woman you married.
This above all: to thine own self be true, and it must follow, as the night the day, thou canst not then be false to any man ~ Shakespeare, Hamlet
When it comes, harness and ride it. It is powerful and empowering stuff.
Until then, focus on not giving him the satisfaction of seeing you sweat. Fuck. That. Guy.
Do you want me to tell you something really subversive? Love is everything it's cracked up to be. That's why people are so cynical about it.
Midlife is a time for evaluation, tweaking dreams, empty nests, and coping with realizing you'ved got limited time left. It isn't a pass to cheat-that is his decision to do. Don't let him hand you any responsibility for his decision to cheat.
I know what he wants. He wants to have you and have his whore. He wants life to go back to the time before you knew. Not that you know about his betrayal, you have made his life "hard". So he wants time away because it will be easier on him, and in the mean time he will try to figure out how to keep you both. BTDT. Mid-life crisis, my ass.
He already tainted and threw away 20 years with you. It will never ever be the same, even if you do R. But to R you need a remorseful WS, not a coward who is looking for an easy way out. NIK is right, the anger will come soon and when it does you need to harness it. It will allow you to put your bitch boots on and allow you to be completely firm with WS WRT your boundaries. He wants you? Then he damn sure needs to win you back now. And that starts with quitting his job and going completely NC with his whore. That is step one. If he is not willing to do that then you have already lost him. Or I should say, he has lost you.
You deserve to be respected and him having an A with his freakin boss is a huge disrespect to you. Him lying to you before dday was a huge disrespect to you. Him continuing to lie after dday when you gave him the gift of R is really the ultimate disrespect. You deserve so much better than that.
If you think you might need anti depressants or anti anxiety to get thru this time, then go see your family Dr. He or she can prescribe something for you if you feel you need it. I know I did! There were a few years where I took a xanax if I needed to see or talk to X. It was the only way I felt confident that I could control my emotions.
You will get thru this. It will not be easy or pretty, but it is doable. You are stronger than you realize, trust me.
This is somewhat like the 180 that is always discussed here. I can't imagine that this is not similar for non-SA situations as well. Don't make threats you are unwilling to follow through on. He wants his space, let him have it. You can't control his behavior. Best case scenario - he returns and you guys work on R. Worst case scenario, the separation expedites the dissolution of the marriage. If the marriage can't be saved, you'll know it sooner rather than later. I say this, in the end, is a good thing. Good luck! Stay strong. You do sound incredibly strong.