Hi,
I will address just one of your statements, that he is very successful and intelligent, and, is about to be involved in a 4th divorce...I can relate to that...although I am not an addict, I do not have the ability to make a marriage work.
My first priorty has always been my career, I married because it served a purpose for me.
I never should have married, period, I could never serve two masters, all my intelligence and success could stop the house of cards from falling down, over and over again.
All you can do right now is to try and think on your feet and work through this dilema. Because you love someone doesn't mean that you have to dedicate your whole life to them. You have not failed him, he has failed him.