i don't know why it's being done. but the advice i get from every counsellor is that when one of you wants out, nothing can be done.
Not knowing why can be frustrating. Normally the WAS can't bear the emotional pain of staying vs the emotional pain of leaving. Leaving becomes the better option. So they cope by having a A, rewriting history, script, fog, etc. You are only responsible for your part of the problems in the marriage.
You sound like a very grounded, secure and capable woman. He feels a lot, he gets other peoples feelings and he is very aware of how others feel. Co-dependence helps you fine tune these skills. He really wants to be needed, his Mom and Dad need him. You are independent. You both wanted children, a area you needed him in and it didn't work out. This is crushing to him. This IMO is a huge source of emotional pain for him. Co-dependent people have a hard time expressing their own feelings. Especially their own wants and what hurts them.
I was very co-dependent. I had to deal with my FOO issues. I had to do my work. I so wanted to feel better about myself and be loved for who I was. To let someone down is awful to a co-dependent. I didn't think I was lovable if anyone could really see inside me (intomesee). I share this to maybe help you have some compassion for your H. Compassion is the solution for anger.
Your H's parents are feeding the co-dependence. They cause him emotional pain - shame, fear, guilt. All negative emotions. That is his problem. You can't control his parents or him.
As much as this hurts it is a time of growth for you. A story that helped me is the Stockdale Paradox.
Cheers Coach
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.