Overreacting to pain? Is there such a thing? I have been reading the thread and I really wonder - did you want him back? It sounds like you were more upset that he left first vs. leaving. You had second thoughts. So did he. His led him elsewhere. I'm not saying that what he did or does is right. It sounds like he is not handling things well. It may be that he had an addiction for a long time. Allen is right that a cheating spouse uses rage as a way to process guilt. Clumsily. The LBS pays for that adultery until they know what happened. Then it makes more sense.
But it's not that clear in your case is it? You said you did something, he escalated in retaliation. You changed and feel he should too. Really?
The custody? If it really is in the best interest of the child, then by all means go get it. But the rest of it? It sounds like you both hurt each other enough for a lifetime, no? He just wasn't able to get over it the same way.
Maybe I'm misreading this. And I certainly do not condone adultery, but let's face it, he was "done". Had no furhter ties in his mind. And he was deal with his pain as he knew how. Just as you dealt with yours as you knew how. It would be nice if you both grew up and faced your issues together. It may be too late for that from the sound of it.
"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."