It does not have to be an affair, it could be a MLC or he might not have the spine to explain himself fully so he "wont' hurt her" (just BTDT) My H was also 100% sure our M was a lie and never loved me and that there was no turning back. I call it the anger stage, nothing you say will change his mind for the time being. I know you feel powerless but understand that he needs to go through the process of letting his anger run its course.
Have you suggested MC? are you seing a C yourself? I highly recommend it, you both need it, if he won't go with you go alone.
DB by not pursuing him, by giving him space, by being agreable and be the "greener grass". Work on yourself by doing stuff you maybe stop doing long ago because of the kids. Before my H left I had sort of let myself go, wore unappealing clothes, no make up, didn't work out. Our self steem suffers when we go through this kind of things, do whatever you think would lift your spirits, work out, find time to do something just for you, coffee with old friends, find that independent person you used to be before marriage and kids. Pleanty of times we forget we are women and act like "mommies" all the time.
GAL
Be not afraid...I will repay you for the years the locusts have eaten Joel2
30something 2kids survivor of S, MLC, A, D I have peace in my heart, at last.