It's likely that your husband was grappling with mixed feelings and perhaps even feeling a loss (as outdated as that sounds) with the reality that his ex was finally replacing him with a new spouse.
It's unlikely that he was looking to genuinely reconnect with her and start over in any way or he would have done that a long time ago. This was much more about the finality of their divorce and her moving on and the impact it had on him.
Working Through Feelings of Betrayal
That said, it sounds like the fact that he has minimized your feelings rather than relate to them has increased your anger and feelings of mistrust.
Related: The Real Reasons We Cheat
How about letting him know you would like to be able to talk openly about what the reconnection meant to him, since it's important to you to understand him— and telling him that it would be very meaningful if he can at least relate to your feelings of distress by the time he spent dealing with his ex.
It sounds like he's feeling blamed and accused by you of being unfaithful, so rather than hearing how upset you felt and responding to that, he keeps reasserting his innocence and that he hasn't done anything wrong.
Before you put the other foot out the door, why not try talking it out with a marriage counselor who can help you get past your anger and help you relate to each other in a caring empathic way, rather than a blaming one.
You've been married along time… it's worth a little work to get through a rough patch.
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