Not your fault-- not even for a minute
His fault. 100 percent his fault. He was an adult, not a child, and you were his girlfriend, not his mommy, and his ex-girlfriend at that. It is not incumbent upon those of us who love alcoholics to save them. We can't. It's not incumbent on us to change our lives to accommodate them and their needs. We can't and be happy.
It is incumbent on us, and alcoholics too, to do what is necessary to take care of ourselves so we can be present in the lives of those we care about (as long as they aren't toxic, then it's incumbent on us to not be involved with them even if we love them).
I support two people and help support four others-- if I take energy to deal with the alcoholic/addicts who surround me, and they do surround me, it takes away from those six people who deserve everything I'm capable of sharing with them. Two of them are children and there is nothing, not one thing, more important than properly caring for children.
Good luck to you.
Cyranoak