You're doing brilliantly OP and it's good to see the clarity of your thinking compared to your initial posts.
My ex left very suddenly and I think it left me with a bit of ptsd. Took a long long time to get my head in a better place and sort through the swirling constant thoughts on why? What could I have done better? When would I hear the reasons from him? How could he throw this all away? So in no particular order and hoping it helps you, here are some of the conclusions I came to:
He has to hate me otherwise he would have to look at himself in the mirror and see the truth of what he's done.
Discovery of the other woman made me feel better because it meant I hadn't driven him away with my sheer horribleness.
I could have been kinder to him. Similarly he could have been kinder to me.
We will never have an honest conversation where he tells me the truth or apologises for his behaviour.
Texts, communication audit trails in respect to "evidence" don't matter. The divorce court does not care how despicably someone behaves. You can divorce him for unreasonable behaviour, one example and the most egregious being that he packed a suitcase and left the family home because you offered to go to a rugby match with him.
Expecting and asking nothing from him in terms of parenting is easier than hoping/ asking and being let down.
Don't be sad or feel guilty on behalf of your child. This one is really hard but you bring your full self to your child's life and your husband leaving and the impact it will have on your son's life is ON HIM not you.
The last one I can think of for now and which has really worked for me over the years is that silence is its own response.
I know how very very painful this is, it's like the Bear Hunt story, you can't go over, under, or around it, you just have to go through it.