I have no advice to offer, only compassion. What an awful discovery. I am sorry you are hurting. I am even more sorry to read that you are blaming yourself. Please, please do not feel stupid. You are not stupid. You are a loving, trusting, good person. You were committed to your M and your WH and you assumed, as a married person should be able to, that he was too. You could not have known, you could not have prevented this, you are not to blame, and you deserve so much better. This all about him.
Be good to yourself today. Be kind and forgiving of yourself not having known what he was doing. And if you decide not to try or if you do give it another, know that either way you are going to come out of this even stronger than ever.