I'm not too clear on what what you "knew" intuitively, rather than cognitively. It sounds as though you have always "known" about at least one affair, but that this was never acknowledged properly. So you've been locked in a dance where you both "knew that you knew". The fact that you've stayed and haven't acknowledged this as a truth has perhaps given him permission to carry on.
I think it would be really helpful to establish what your bottom line is. What could he reveal that would mean you couldn't stay? You see, if you feel intuitively that you don't want to be confronted with something that will make you leave, then tread carefully. But the consequence of that will be what I would describe as a "veneer" marriage. Perhaps too, that is what you've had all these years, with these massive secrets and unacknowledged truths. That is why this discovery is simultaneously an opportunity and a threat.
I can imagine your pain because perhaps you have been bargaining all these years that there were no deep feelings involved and that it was just sexual release.
This is a scary time because all of those bargains you have been making have been like a protective shell around you and were necessary to survive in this marriage. Confronting this now also means confronting your own collusive behaviour down the years. If there is anything in you that wishes you hadn't found those letters or that he hadn't kept them, then that means you've been bargaining.
I would always advise knowing everything and having complete honesty, but that's because of my personality and how much better I know a marriage can be, for having complete openness and transparency.
So I think you need to be pretty self-aware about your own personality and whether you can cope with the revelations that might ensue, what sort of marriage you want in the future and how you want your life to be.
I don't think there's a half-way house about this either. You either want to know it all or nothing at all.