Men typically don't remember as many of these details as women and they also like to "forget and move on" whereas women need answers- we tend to need some type of closure to move on He doesn't want to answer because he doesn't want to face his mistake by giving you the details and reliving it. Too bad. He messed up- he needs to pay the price by coming clean for YOUR sake. It doesn't seem like he put your needs first when he did that so tell him he needs to put you first but answering the questions Honestly he may not remember dates (it started and ended) but he can remember many of those other details.
I strongly advise finding a marriage counselor to get through this. You could always go to the first couple appointments alone to make sure YOU like the counselor and then you both need to attend. Best wishes.