You didn't do anything wrong!
Remember.....you didn't Cause it, you can't Control it and you can't Cure it.
Why did he all of a sudden stop when he met her? Who knows. The grass is always greener on the other side (ie a new relationship for him) so he's in the honeymoon phase where everything is all daffodils and rose coloured. Hopefully he is clean and will continue to do that, but if he's not doing it for himself (and doing it for his new g/f) nothing will keep him clean but him.
Don't worry about him any longer. You've made (in my opinion the right) decision and are sticking with it. Let him live his new life how he sees fit, just don't enable him in any way, shape or form.
I'm the queen of enabling (no longer since I started AlAnon last week) and totally feel for you right now.
Continue on the road to bettering yourself. You totally deserve it.
Rebecca