JRG- Wow, that sucks. I'd be torn up inside too and angry, sad, confused-- the whole gammut. From the outside I can say "it doesn't surprise me" but if I was in your shoes tha would be cold comfort. That has to hurt more than anything and I am so sorry.
I have had my suspicions (long before we separated) with my H. Not anything physical mind you, but emotional. I think that when we stop playing the game and they might have to look at themselves, they hurry up to find someone new who will see them in whatever light they portray that day so they can feel good (or less empty) about themselves. I don't suppose it makes it hurt any less, but it definitely is not about you that he's gotten himself involved with someone else (I am just going on the assumption your gut is right bc I've learned that my gut with my H is right 99% of the time and I bet yours is too). He's empty and doesn't want to deal with life on life's terms or reality or do the hard work to live a real life. So instead he's going to place responsibility for his happiness or lack thereof on people, places and things and thinks that by replacing people, places and things he'll find the answer. We both know he won't but that doesn't make the sting any less.
I know when the day comes that I learn my H is involved with another woman it will be hard to swallow. I still love my H but can't be with him and maintain my health and sanity so letting go of someone you love and the life you thought you'd have together is hard. Then add in his seemingly blase attitude about finding a "replacement" and that is salt in a wound.
I am so sorry you are hurting. I would be too.