It is so hard in the beginning. I feel like we are similar, in that we blamed ourselves. I feel that your husband used very similar tactics that mine did....
They do something wrong. We react. They stay silent and unemotional. We look like the bad or crazy guys for reacting with emotions.
This is my advice.
1. Mirror his lack of emotion and way of communication. Keep it polite, detached, and only related to your child. Write knowing that lawyers might one day be reviewing your writing. You do not want him to look like the calm, rational one while you look irrational. (Higly recommend reading mustard seeds threads. Her husband cheated,and abused and instigated and used her reactions to get a restraining order and limit custody)
2. LEARN TO BECOME YOUR OWN ADVOCATE. You are going to have to learn to see things for what they really are. It is hard to stick up for yourself when everyone is telling you that you are wrong. You are not wrong in this. Your ex is wrong, OW is wrong, his family is wrong. Trust your deeper voice that is angry for you.
3. Do not expect or even bother to attempt rationalizing to your ex and his family. For whatever reason, they cannot or do not want to see your perspectives. There are tons of diagnosis and reasons for people like this. You cant win with them. They are incapable of it and it will just show them your weakness. JUST KEEP IT BUSINESS AND FOLLOW LAWYER ADVICE.
4. Give up hope for making a marriage work with someone capable of doing the things he did. Life is filled with hardships. Your ex prooved he cannot endure hardships and then blamed it on you. Life will inevitably get hard again and he is not capable of being there. OW is not getting a prize. Because eventually he will screw her over too. She cant be easygoing and fun all the time. Its just a matter of time.
Its actually a blessing that you are learning this now while you are younger and self sufficient and healthy again.