I haven’t read all the comments above, so forgive me if someone already said what I am about to say. I read up to where you said you are 6 months pregnant. Don’t assume he left you because of her age and your age. It belittles the both of you. It makes it sound as if you are less than her or she is less than you. Truth is, he is less than the both of you. She didn’t get a deal when she got him.
She is no prize because she probably knew he was married when she persued the relationship or at the very least didn’t end it when she found out he was married. She will always feel insecure every time he is around a pretty woman. She knows that she is getting a man with no morals who is selfish.
He will never again be in a relationship where he can be trusted. The real question you should be asking is :Was the marriage a failure because you both were not ready or was it a failure because he was not ready to share his life with anyone.
Look for answers but not for blame. Only look for answers so that you can move on. He is going to be the babies father so you have to find a way to make peace with it. The life you carry will always be a part of him and that child needs to feel love. If he choses not to be a dad, than, just collect child support and love the child enough for two people. The rest of life will sort itself out.
Short answer to your question is, He left because he didn’t love you enough and have enough strength of character to stick around and keep his vows and make things work. It was simply easier to go to someone else who was desperate enough to ignore his flaws.