This is a very painful situation, and there is no easy way through it. It hurts, you long to have him back, all the time and even is sleep you probably still feel the loss. It will get better over time, unbelievable as it may seem, and one day you will remember how devastated you were without feeling anything much at all.
Right now you have to concentrate on getting through the days, not brooding or ruminating for more than a few minutes twice a day, and keeping to a healthy lifestyle. This may mean having a rigid timetable for a few weeks or months. Take exercise, start a new hobby, get involved in a charity that interests you.
If your marriage were a hundred per cent, would the affair have started? Usually there is a missing ingredient, and it could be a good idea to start thinking about that. There are some books on these and related topics. You could browse www.amazon.co.uk and see if anything there appeals to you. best wishes.