You don't have to apologize for being angry at your ex at all. There is such a thing as justifiable outrage.
To put in the most simplistic terms your ex is an arsonist.
He keeps setting fires to your world. When he chose to have an affair, get that woman pregnant, move in with her and have a family with her...he burned your world to the ground and there was nothing anyone could do but watch your future burn.
If someone maliciously set fire to your house no one would be surprised if you refused to forgive the arsonist.
That being said, I think you have handled this entire situation remarkably well. My husband was an A$$ but your husband rules as his king.
Your husband is a pretty mean guy. I say this because the lack of empathy he keeps showing for how he has destroyed (and continues to destroy) your family's life.
Case in point: His most recent conversation with your son. It was unconscionable. After your husband destroys your life with his affair he has the unmitigated gall to defame your role in the marriage.
He continues to display an unbelievable lack of character.
I do not care if he thought you were an unloving wife. Chronic harpy. Frigid lover. Or compulsive spender.
He cheated on you. He brought another woman into your bed and got her pregnant. He broke the most sacred of your wedding vow and he knows this. If he had an issue with your behavior then he should have addressed it. Working through problems in a relationship is expected. What is not expected in taking a lover.
You need to understand that your anger is justified.
A terrible wrong was committed against you and it needs to be acknowledged.
Whether you realize it or not what you really want is your husband to acknowledge this.
AND HE SHOULD ACKNOWLEDGE IT.
But he won't. And he won't because he is a self-centered child.
How do I know this?
Because he is blaming you for the destruction of the marriage---when he was the one setting up house with his lover while married to you!
Be angry. But in your anger also be wise and see him for what he really is.
And by the way: Your response to his text was spot on.