Oh Misery, I'm just so sorry to hear of your struggles. We've been there, done that, got the damn t-shirt (like we'd EVER wear it!)
Reading your last post, I wondered if Al-Anon may be helpful (or CoDa). I am NOT saying you are co-dependent... I am saying that some of your posts describe things that helped me via Al-Anon and Melodie Beatty's CoD books. One is that awful guilt and the worry about "kicking when they are down". It took me a long time to "get" that my choices do not "make" anyone else feel or do anything. Those are THEIR choices. We are only responsible for our own choices, and we are also, IMO, responsible to ourselves (in that put on our OWN oxygen mask first kind of way).
Mid life crisis, mental health, complete change in life, or whatever.is going on with your WS, he is the one that CHOSE to have an A. And he is the only one that continues to CHOOSE to contact his AP, despite being well aware of the harm it brings to you, and (i assume) the potential consequences to the M. You continue to CHOOSE to fumble trying to an oxygen mask on HIM before putting one on yourself. Trust me, I am "guilty" of this in many ways (still, tho not WRT my WH).
Put simply, I don't think that works... in life generally and absolutely when it comes to infidelity. There's a great interview with an IC named Stan Tatkin on the Helping Couples Heal podcast in which he basically says the post dday relationships that have any hope of working are those in which the BS sets clear boundaries and STICKS TO THEM. You are not the 1st (and sadly, will not be the last ) BS to struggle with this.
You don't HAVE to decide right now if you are willing to wait the years it may take for him to figure/sort himself out. However, I think it would be helpful if you worked on figuring out their boundaries you need TODAY and then stick to them. It sounds to me that continued contact with AP is one of them for you. So figure out the consequences that YOU can live with (not him - YOU), communicate them clearly (and going to his IC where he feels "safe" - yeah, feels so unfair, but all too often true - may be a good way to do that communicating). That is not "focusing on the infidelity", that is protecting your own heart from further harm.
If your WS is not willing to 1000% commit to ending ALL communication with AP immediately and doing so without any wavering (including a NC letter with threat of legal action if she contacts him, deleting all contact info, blocking on phone, at work, email, etc. and allowing you to monitor ALL his communication and electronics - including work- until further notice...which could be forever), then you implement your consequences. Not to coerce or manipulate or change him (this is NOT about him!), but to protect yourself and to provide some "breathing" room for you to wrap your head around what's happened, what's happening, and how you will learn to manage it all.
If he can't do that, then he can't (or won't) do that. You know where you stand and you will have sorted out how you wish to proceed with YOUR precious and only life.
None of this is "selfish"... IMO, there is a HUGE difference between self-care and self-ish. Again, this is NOT about him or trying to make him into his "old" self.... it's about YOU. Same goes for the 180 or grey rock or any other boundary. Boundaries are NOT to change the other person's behavior... they are to keep one's own sanity. An alcoholic can go binge drink and do their thing all they want..... but they cannot have their booze AND have me in their lives. Same for your WH. He can have all the As in the world he wants... but he cannot have his AP AND have you in his life, any more than I can scream at my kids over every little thing and then expect them to trust me when they need to call me from the police station (or whatever - you get the drift).
What helped me may not help you. That's OK, but I would emphasize that you continue to search for what WILL work for you to help you move through the [temporary] place you are in right now. What do you need (in this world, not the "shoulda, coulda, woulda" world we all wish we had the time machine for)? What do you want? If you can only change yourself, what can be changed? What tools or support might you need?
Sending Hugs and strength, and looking forward to a day when you can change your username - that "misery" does NOT last forever.