I'm so sorry you're having a hard time with this. don't have any advice about whether or nor he's having an affair, but i do know that i've been where you are, only mine may have had an affair. i try not to allow those thought to take up too much space in my head. time will tell you what you need to know.
i think that it was good that you were able to confront him, i know that you still don't know whether you can trust that he's telling the truth though
to keep from obsessing, maybe you can replace those negatives thoughts with positive one, whenever you have those thoughts. sorry but even if he does decides to have an affair, there is not much you can do to stop him if thats what he really wants to do, you just don't have to stick around until it finishes making you sicker.
when i first came around, i had a hard time figuring out what was/was not real too, and it think checking cells and emails is kind of common codie behavior, that will soon pass as you began to focus more on you.
it helped me to decide what my gut was telling me whether true or not, except it as true, develop a plan of escape, protection or whatever else you think that you might need to make your life better and have it ready just in case you need it one day.
sorry for the book, think i'll stop now and pray for you and your family