Hi ssarah, So sorry that you must be here but with what you are going through, it's the best place to be. Just so you know it's not just men who go through the same script. My W has said the exact same things, acted the exact same way. The way that they say one thing one day and then you get a letter from L and it's right back to all your fault is also very common.
The biggest thing to remember is that you didn't cause this. You didn't do anything to make him feel the way he does. I bought into all the weird stuff my W said was the "reason" she felt the way she does for so very long. As soon as one thing changed, she would just find another, different "reason" this is all my fault. One day things were great, she would talk about the future, the next things were hopeless. As for the OW, she is just a symptom.
My W's parents went thru a very bad D. She always swore that she would NEVER get a D, that it was for weak people who didn't want to try. Now she has filed and left without ever really trying. They will do the things that they always said they would never do, get angry at anyone who tries to talk sense into them (like his brothers) until they start to come out of the MLC tunnel. The best thing you can do now is to GAL outside your H. Become you're own person that has a life apart from him. I know it's not easy with the kids around and the pain you are in but you really need to work on yourself.
Keep posting and hang in there. There are many on this board who will help you in so many ways!