Like you I am ashamed of how I prioritized him over her ... .Had I had the chance, there was a time right after the breakup when I would practically have given her up just to be back with him, I was that destroyed by him leaving, and that addicted to him.
He was clearly triggered by the (mis-) impression that I was choosing her over him.
Yes, I agree, he was my addiction. I remember once, when we were having problems and I thought he was leaving, I was physically sick. Never before have I reacted to anyone in the way that he made me react. It was an actual physical addiction - not just emotional and mental.
I also agree with your observation about the choice thing being an issue. My ex even told other people that he thought I was choosing my son over him. They advised him that this was normal as he was my son and I would never turn my back on him. Unfortunately, my ex could not accept this. He often argued with me over this and although I tried to explain that I should not be put in a position to have to choose, he just put me there anyway.
I still think now, that him leaving was his final attempt at making me choose him. But it backfired on him really as he was left with the choice of a life alone or with the replacement and a whole new set of problems. I can't help but think that he didn't really want either of those options but things had gone too far by then. And to really top it off for him, he did me a massive favour and put me in a position where I have been able to rebuild my relationship with my kids, better than it has ever been. He tried so hard to destroy my family and failed miserably.