How utterly miserable for you. I'm so sorry your husband's being such a twerp! Of course he's all calm about it; he's had ages to think about it, it's his decision and he's confident your family will be there in the background.
I echo your other respondents. He's not had to face humdrum reality with OW yet, and he won't even have considered that her little ways and habits will be different from yours. His shock will hit him later. By that time, you'll have got your own new routines in gear and be well on the way to a whole new, happier and more fulfilling life. If you want him back, this is the way to go.
It's also the way to go if you don't want him back - either way, the secret is to PUT YOURSELF FIRST although I know that feels ridiculous at the moment. Please start immediately: ring your women friends, sort out some irresponsibly alcoholic evenings with them, book a new hairdo and buy some new & different clothes. Take very good care to eat and drink enough - you're allowed a diet of chocolate and Tango for a few weeks (whatever you can stomach) but then start treating your body as a temple. Get some sleeping tablets if you can't rest.
On the 'getting him back' front: the most reliable method is to build a happy, interesting, new life as quickly and thoroughly as possible. Don't let him hang around the house, he's got to get out of there and start being separated pronto. But invite him to all & any family/friends events, days out & suchlike, without OW and without commitment. That is, lay him a place but don't put any pressure on him. If he turns up, be pleasantly normal but don't even hint at any relationship issues. Just be yourself (albeit emotionally dishonest). He mustn't feel he can just wander in whenever he likes - change the locks - but do invite him.
The beauty of this approach is that, not only is it the one most likely to work, but it also sets you up for a great start if it doesn't work or you change your mind.
The best advice anyone can give you right now is to take your OWN well-being very seriously indeed. It's a big switch of focus, but you can do it.