Oh, spook,
I've just seen this thread, and I feel soooo sorry, I know I'd be devastated in your shoes.
You've said how much you love him, and how much you want him back, but does he know what he wants yet? Obviously it would be ideal for him to have things as they were a month ago, but that isn't an option. It's no good you wanting him to come back if he can't settle. I guess that, for all the things running thru your head each evening, there are things running thru his too. I believe men in this situation do feel guilty (how can they not), and as they so often do, men deflect that off you, the dw. The guilt gets in the way.
I think you need to spend some time (like a weekend) together without your boys, so you can really talk, without a time limit, and without the worry that the boys will hear if you shout at each other (which I think would be pretty reasonable in the circumstances). I wouldn't try to understand why he had the affair, I guess lots of men in there 40's don't reason it out, they just see an offer and take it, flattered that they have still "got the magic", especially if his business and personal stuff (like his friends with cancer) seem to be going down the tubes.
What I would ask him if if he still loves you. NOT in comparison to her (they are probably different feelings anyway), but all things put together, does he picture his future life without you? only seeing the boys at set times. If his business doesn't survive, who will hold his hand and help him thru (bet she won't, when he needs propere emotional support and he's not got much cash to fling around). who was there helping him build it up in the early days?
I have two phrases which I use when dh and I are at odds with each other.
"I don't need a great deal of love, but I do need a steady supply " is one.
If we're really rowing (doesn't happen often I'm pleased to say) then I tell myself
"I'm beautiful, and I'm bright, and I deserve better than this".
At the end of the day, if he decides to go, then God that will hurt. But it will be an end. You can start to organise life for you and your boys, begin the grieving, and begin to look to the rest of your life. If you let this drag on, you will just open the cuts again every time he rings, or comes to see you. That drags you down, and will affect your boys eventually.
Sorry, I've gone on a lot, just wanted to give you my support. Love to you and your boys. I hope he sees sense and comes home, learns again to love those who really love him.