You have had a 2nd DDay. And he is a gigantic horse's patoot.
Now gently, if he goes through with plans to see AP - change the locks. If AP has a spouse/partner - EXPOSE.
To answer your specific questions:
How did you all manage to keep going through the day-to-day motions?
One foot in front of the other, crying in private a lot. But I kept f***ing going.
Did you share what has happening with your boss at all?
In my case, it got to the point where I had to. It was done confidentially and he kept my confidence.
Go on medical leave?
I did not. But there is no shame in doing so.
Just power through?
Yup. White knuckles and all.
Now - a few corrections to your post:
How fucking stupid of me to trust him, when he already betrayed my trust so fully.
Wrong my friend - how fucking stupid of HIM to betray your trust AGAIN!
We were trying to work through the betrayal and reconcile.
Gently, you were. He was still getting his bread buttered on both ends and liking it.
Our 12 year anniversary is this weekend.
He knew this and still behaved as he chose.
losing this last chance to be a parent.
My dear, this is 2026 and fast approaching 2027. You have options. They just may look different than your vision. Currently, he is not close to co-parent material with you.
I have a pretty person-facing job, so I have to constantly put on a happy face and act like everything is fine, which is exhausting.
I feel this. For the longest time post DDays I pinned a meme of the famous Elizabeth Taylor quote to my mirror, my car, my desk, my monitor, my vanity "Pour yourself a drink, put on some lipstick, and pull yourself together" My drink was coffee, I bought a bunch of new lipstick and matching glosses and sparklie undies for good measure. I wore them a lot as they gave me a confidence boost. I still do. Find your version of that.
I feel like I barely have the capacity to fold laundry right now
So don't. There is no Laundry Police. Especially don't fold (or wash for that matter) his.
Be gentle with yourself Shrug6645 and keep posting. You are in good hands here. Hands that will hold you up when you don't have the energy to hold yourself.