I am so sorry this happened to you...please accept that from me.
I am curious, and I do not mean to offer false hope, but are you certain that this is the case? It seems to me that he would've never mentioned it had he wished to keep it from you, thus I am wondering (and hoping) that perhaps what you found out last night may be wrong. Again, I do not wish to offer false hope nor do I mean to seem like I'm doubting you, as that is certainly not the case. I only wish to be sure.
That being said, if I were you, I would confront him about this. He owes you an honest explanation of what happened. As far as long term goes, that is up to you. You must understand though that if you choose to try to work through this, it is going to take time. Forgiveness does not mean that the emotions instantly vanish. They will be there for some time. Both you and him would need to acknowledge that, and you would need to be patient with both him and yourself as things try to heal. He would need to be patient to you, and he would have to do everything in his power to rebuilt what he has destroyed. Additionally, understand that trying to fix it is not a guarantee it will work...it is only trying.
I implore you to talk to him...get all you can out of him regarding it. Give yourself some time to cool off and to think. Don't make a decision until you're able to process everything with a level head and all the facts available to you. Then, and only then, do what you think is best for you.
Please know you are in my thoughts and prayers, and that I wish you all of my best.
Hugs,
Harley