Hi Hope. Happened by about the same time you posted. The dilemma - hopefully it helps to articulate it.
Addiction? You can't make him stop I suspect. You can confront and bring it to crisis, but only he can stop it sadly.
The OW? Does it matter if the addiction is there? Accept that he is finding happiness? I've never known a happy addict while they were using. But your choices are more about you and your son at this point since you really can't get husband to change if he doesn't want to.
How to proceed? Only you can answer that since you have to live with it. But I think it may have been helpful to put it all in perspective. At least I hope it was.
Regardless of what you, your son deserves to be safe and to have a relationship with his dad - in that order. In my opinion.
Your ability to love and be loved? I think you have to answer that. You have to know that you are who you are and that what got you here is the past and not your future. You have to figure out if you made a mistake in choosing him, or if he changed along the way in ways you could not tolerate. So that you know if you need to change what you look for in a marriage should you go that route.
"Our lives begin to end the day we become silent about things that matter" MLK Put the glass down... "Yesterday I was clever so I wanted to change the world Today I am wise, so I am changing myself."