Hello Tombplant,
I'm so sorry you are in such pain. Losing a relationship is a kind of grief, and it will take time to heal. You are so fresh from this break up. Of course it is all you can think about. You are not responsible for his relapse. That came down to his own choice.
This site is littered with the stories of relationships falling apart. It sounds as though your ex has a lot to work through. I know you are probably hoping that I will say that his behavior is very common and to just give it time. He'll come back to you--but I can't. His behavior is common, but it can be the start of a merry-go-round of breaking up and reconciling that will be even more painful than what you are experiencing right now.
Please give yourself time to grieve and heal. Treat yourself with kindness (don't let that tape in your head continue to call you names). It gets better with time. I promise!