Stardust,
You are a very smart person, unfortunately, you are also proud and this has hindered you much of the way: you truly believe it couldn’t have been a bad judgment call from the start. You cannot blame him for lying to you, he never forced you to believe him.
So I have only one advice: the young boy is gone, don’t try to find him back. You must be in love with the man he is now.
Can you do that? Because if you can’t, the other woman can. She has shown to your husband that she can love him as he is. He knows she doesn’t love him because of how he was last time, some false memories he isn't even anymore. To him, she is the perfect one who accepts him as he is now, flaws and all. And now he has been discovered, she has dutifully backed down, whereas you are the one drawing all the rules and boundaries. Of course this is not your fault, it’s just the dynamics of the situation.
To be honest your husband is a great actor, his life is one big drama. To you he was the boyish, protective, caring boyfriend, then the responsible, loving husband. To the woman she was the special one, having met her too late, a tormented lover tied down by a loveless marriage he is unable to leave because of 2 young sons.
How could she (and anyone or that matter) not believe so, when he showed her he would be there for her no matter what, no matter wife was pregnant and alone, false contractions, confinement, in depression even seeing a counselor (even applauded by him as an effort to solve all her imagined problems), waiting for him to come home. And now, we can only guess intelligently what he has traded for her to give him this space for a while.
He has proven to her he loves her so deeply that she holds these memories dearly in her heart, believing that when the storm is over he will come back and do her right. You are human, but she is too and her heart now is aching just as much as yours.
He fought and struggled not to be exposed, but now out in the open, he has changed the act to fighting for this marriage. If you leave him, he may not be able to handle a big stage play to a suddenly overwhelming audience: his parents, colleagues and friends who respect him for being a faithful husband etc. upright man, 2 sons he will be guilty to face for the rest of his life. Acting in a small film and a blockbuster movie with unforgiving critics is a totally different thing. He can escape you, but the unforgiving critics will always be there to torment him.
Looking at your previous posts, I guess you might say deep down he is still the same guy. Yes he is, but for 13 years he had only shown you what was in one hand. Now he has opened the other and showed you what he has kept hidden in his palm all this while. As long as you face If squarely and bravely, you will be able to lead a happy life, whatever decision you take eventually.
Stardust, I don’t mean to be negative or positive, or to discourage or encourage you. You believe that if you do not live in denial, you might be miserable for the rest of your life. That is not true. When you see things as they are, it is always enlightening and easy to accept.