You aren't an idiot. Don't bash yourself with a stick my lovely, you have had enough of that from him already. He will try to get into your head now that you are making a stand. He is a very sick person and he isn't capable of thinking about the hurt he is causing you or your well being. When I was drinking I blamed everyone else for my drinking, it was always everyone else who was the problem not me, I would lie about how much I drank and that I was sober when I wasn't. I took pills a few times to try and kill myself not really wanting to kill myself and I didn't care about what effect that had on the people closest to me, I would lie, manipulate, cheat and it would all be somebody else's fault of course.
He is completely lost in his addictions and his mind is so warped. That isn't an excuse for his behaviour by any means. Only when I became willing to get sober (and I had to be broken to do so) and started working the steps of AA I was able to sew the damage I had caused and was able to begin repairing it.
Just try to take it one day at a time. One day at a time you can focus on yourself and your son and do the next right thing for you and your son. Your partner isn't your responsibility. You and your son are. That's it. I would block the girlfriend, you do not have to read messages from her she has nothing to do with what is going on between yourself and your partner. Are you able to block him and only have communication via lawyers? The less you have ti hear from him or see him the better. If he does contact you do not get back into the ring with him.
It's great you have come here to post. There will always be someone who can respond.