I'm sorry you are having to deal with this. To hear that a spouse loves another....it's just horrible.
Sometimes, like it sounds like you are experiencing, it does become a defining moment. I don't want to paint it as a "good" thing, but it is "explicit", in addition to being painful. Knowing where you stand provides the opportunity for you to start making choices based on real information.
What you know is that he believes he loves her and believes it enough to tell you that. You also know that he likes her more than he likes you. You also know that he blames you for some of his problems and had used that to justify his affair.
I know that I don't get a vote, but if I did, my vote would be to let them have each other.
I am very sorry for your pain. Please keep reading and posting on SI. Good luck to you, as you navigate the next difficult steps of this living nightmare.