My heart just breaks to read your story here, painfully. What a huge disappointment. All I can think of right now is its just not fair to be treated that way, the mother of his children, the woman who stood by him through some pretty lousy years, and just set aside as if none of that meant anything. What a selfish, thoughtless jerk.
People never cease to amaze me, and as hard as I try to hang onto my optimism, I know right now I am pretty cynical given my own experiences. But each day gets better, especially when I stopped having the crazy conversations and trying to make sense of the nonsensical. When I accepted this is the way it is and I want no part of it. Being in the "crazy" mode, as you describe and I am all too familiar with, is just too toxic for my life and my health. When I reached that point, it made walking away that much easier.
Hang in there. Keep coming back! And stay strong for your family right now. Your kid(s?) need you to be the one to set the proper example, since their Dad just completely dropped the ball on that one.
And P.S. Karma IS a bitch (as another poster said already). Chances are highly likely this will be an epic fail with the new GF.