I know you hate to hear it, but you are still in early days. It takes so very long to find your way through this, and even once you do, the emotions take even longer to remain settled and consistent.
As for the "why", it really isn't you who needs to find it; that is your WH's job. I think your job is to either accept it or not, and to decide what that decision means to you--how it drives your actions and thoughts. I will say that understanding my H's "why" was an ever-evolving, ever-deepening growth experience for both of us. And while that growth brings more and more comfort, trust, and day to day awareness, it was never a "silver bullet." Just knowing the "why" didn't bring us back together in the healthiest sense, but what we did with the "why" did help to finally restore us. Additionally, the "why" took one hell of a long time to really discover and work with.
I agree that after HB a return to anything different or less feels uncomfortable; although sometimes it felt like a relief to find some normalcy in all of this. For what it's worth, I have also learned that in my case, our sex life builds on itself. If we don't stay engaged and active with it, my desire becomes less and less which makes our intimacy less and less, which makes worry about our relationship increase, which makes doubt about our future increase, which...you get the picture. For me, I have learned to feel close, I have to be close, but that doesn't always have to mean sex. It can mean spending time engaging in great conversations, cuddling on the couch, sharing laughter, enjoying quiet time on our deck, etc. Sometimes, during the hardest times after Dday, it meant slogging through a tough and meaningful therapy session together, or discussing the A together where my H understood that I needed his help, patience, and complete support--and provided it.
I understand your WH's desire to move on, or zone out, but honestly, that's great for him, but awful for you. I do agree that he may need some time to just sit peacefully with you, but that's a tall order this soon after Dday. Sure, it'd be wonderful if you could provide that for him, but it may be understandably too soon for that to happen; I bet you'd like nothing more than to sit peacefully with yourself and him too. I'd encourage you to look for positive moments with your WH and to bask in them, if you can make that happen. If you can't, it doesn't mean it will never happen, just that it isn't happening right now.
It's really not your job to pull your WH out of anything. That can become a distraction that keeps you both away from your own healing. It's not up to you to comfort or soothe your WH, and if you repeatedly do that, he will never confront his issues or heal himself. Nothing will really change for, or about, him if he doesn't have to do the work.
Fears of renewed contact with the OW are common, as are your thoughts of her. Continual checking makes sense and will decrease every time you find him innocent. I know you're exhausted and sometimes just want out. That too is common, and may or may not be how this will all end up. My advice, again for what it's worth, is to accept yourself and the ups and downs that come with infidelity; you don't have to make any permanent decisions right now, so give yourself permission to take things a day at a time. Also allow yourself to seek and obtain what you need to feel better and heal. Don't sweep things under the rug to make anyone feel better in the moment, don't be silent when you need support or something different than what you're getting. Stay in both IC and MC, and make sure each is moving at a pace and in a direction that you need.
And one final piece of advice that I completely failed to master until year 2 or 3 of R: when you can, try to express your anger in healthy ways. This is not so much about your WH, but about you. When I "lost" it, the shame and ensuing regret just added to my pile of crap to wade through. When I acted in assertive yet honest ways, I was proud of myself and it strengthened me. It also prompted the best results from my H. He couldn't dismiss me as irrational, or rush in to attend to my emotional overload; he was forced to really hear me and to attend to his own emotions and behaviors.
Hang in there NotsureIcan. It really is early days, and you will find your way. Every passing day, while painful, is a day closer to healing.