Mrs M - you are not going to understand, because it is simply not possible to get into their heads. Even if they explain, it makes no sense to the woman left behind. It is a question you always come back to ....and I think all of us who have walked this path came back to it repeatedly. There is no answer that you will ever understand.
Please - if you can - try to benefit from our accumulated misery, and try to put the question in its place, with the other things that are simply beyond our comprehension. Not easy, we know, .... but you have a head start.
I am sorry if I scared you
No, you will not feel like this for the next three years ... day by day, week by week, you will move forward a little. Eventually there will come a day when you don't think about him at all, and then there will be several when he is just not a part of your thoughts. Then you will have a dire day when you sit down and all the old thoughts come flooding back ...... BUT after some length of time (and it was 3 years for me, after a similar length of marriage) most of your life, and almost all of your days, will not be overshadowed by what-if and why's. I am not saying you will ever stop hurting, or hating him for what he has done, but you do stop picking the scab.
As for whether he will ever think: Oh f* what have I done ..... yes, I think he will. At some point, when he discovers the grass isn't really any greener on the other side of the fence, and he has lost his daughters, his decent stable life, and all that went with it. But Mrs M .... if he came crawling back at that point would you really want him? really??
I don't think so, and that is the element you have to hang on to at the moment. You want the "good man" that didn't do this ... and, as I discovered with mine, they didn't exist. They cheated, lied and schemed long before they left. You wouldn't really want the reality if he came back and offered you diamonds.
Hang on to those feelings, and guess where I am going to tell you to stick them 
As for feeling guilty ..... God Mrs M I would like a penny for every time I thought it was all my fault. If I had been a better wife, a better lover, less focussed on being a mother, less active and more at the kitchen sink ..... you can beat yourself up for ever, but the fact is it wouldn't have made a blind bit of difference. We were in it for life ...they weren't. How could anything you did or didn't do, change that.
Like me, you say you had no real inkling this was coming and no knowledge that he was cheating. If we were really screwing up our marriages, wouldn't they have sat down and talked about it. The only thing you are guilty of Mrs M, is being conned into thinking he was one of the good guys Like me. And Skye.
Mumsnet jury says "not guilty"!!!!!
Have some flowers for winning your case 
x x x