You're exactly right in knowing you can't choose who you fall in love with. I never in my wildest dreams thought I would fall in love/infatuation with a churchy girl, anything but. But thats beside the point...
All I can say is, you poor soul. Although I myself have never fallen in this situation, I can say my parents have. So I can only give advice based on their input and my watching them before their devorce. Both my father's (biological and non) both have had histories of drug abuse. Before they divorced, my (bio) father had been convinced of not drinking for 13 years. My step father has and still is off of cocain since he met my mother. So what I CAN say is that it CAN be done. As far as the smoking, thats not impossible to live with, as its the mildest of all the drugs out there. I wish you the best of luck for you and for him. You both are in for a rough ride, but it will pay off in the long run. Just remind him every once in a while that he doesn't have to do drugs in oder to have fun (since most drug addicts believe otherwise)...
I hope I have been some help to you!
It is impossible to love and be wise
-Francis Bacon