Hurt mom, You're so right about life as you knew it being over! This isn't supposed to happen to anyone! Yet, here we all are. And people join this board every week. It's so sad.
I second what LWB said, too. Take care of yourself and your little one. You probably have no idea how you're going to do this, but life goes on.
The A is a fantasy. He was having problems with the responsibility of raising a child, but OW has a child? He's not going to find what he needs there. Be the person he wants to be with. Let him see what he will be missing.
But also think about your own boundaries and what you're willing to take or put up with.