Hello there.
I'm so sorry for the pain and anguish you may be experiencing.

It sounds like you've had trouble in your marriage since the beginning? You said right after you got married, things went sideways. I am not sure what that means exactly, but it seems like there were problems early on.
Perhaps from that perspective, this isn't as much of a surprise? Why don't you believe him when he told you he's been feeling like this for a while? Did his actions show the opposite? Granted, he definitely should have waited before getting involved with another woman and you should have separated first, out of respect for you and for the marriage. That only causes more pain for you.
Is divorce definitely on the table? If so, I would figure out the logistics of an official separation and divorce. Do you have a support network? Do you have friends and family you can lean on, even if they're out of state?