Be kind to yourself. Take walks, long baths, go out with friends, distract yourself,let him look after the baby/child(?). Buy yourself little treats with NO GUILT.
Your aim is one day to make yourself the strong woman you once were and can still be. And some. Not for him or to be competing with her or anyone else, but for you.
Someone on a thread here in a similar raw state is managing to keep her (inevitable and very important) questioning to a set time each night, she finds that helps keep it less volatile. In between times write down your thoughts keep a diary of what you are feeling and what you need to ask him.
The going over everything in your head is I think quite important, it is part of the getting over it. At present you are in a state of shock, it is natural to worry it and not be able to think of much else. Gradually you will stop doing it, it will lessen quite naturally, but you need to make sense of the whole thing. Just don't torture yourself with it. Watch the thought coming, think about that thing, then put it aside.
By the way I was angry much of the time I raged at him in the middle of the night sometimes and we each said the most awful things but sometimes we also got close in a different way too than we ever had before. In those close intense times you get a glimpse of why they say an affair can make your relationship better than it ever was. But you both need to work hard to get there and stay there.
Do you love him? Does he love you? Most importantly has he stopped seeing her and cut off all contact with her?
Take yourself to counselling. It is not fair that he won't go but he is probably scared. There is a woman on here called WWIFN who will give you good advice about how both parties can look at the build up to the affair and how it happened, but he needs to listen too.
I used to be very anxious before the affair, but actually in a funny way it freed me from it.
Shit happens and we can't control it, or stop it but we can make ourselves strong to cope with it.