((Indigo)) We all have setbacks/backslides. You can't undo what you did, but you can forgive yourself and move forward.
I am curious, is in he recovery for his 'addicition'? Is it only that you wouldn't let him smoke in the house? If he is NOT in recovery, this may be the time he needs to work thru that. Also, unfortunately, I agree w/kml that it sounds like there may be an OW. It doesn't *really* matter, per se, only that when a WAS is in the 'fog' of a new relationship, there isn't much you can do except work on yourself and become the 'greener grass'.
Going back to the substance abuse issue-have you gone to any al-anon meetings? They may be good for you.
I don't really have any great advice accept turn the focus back to you, check out al-anon or CoDA, and do NOT beat yourself up for a backslide. If he is still using, you probably don't want to get mixed back up with it anyway. He needs to hit his rock bottom.
Me-43 H-46 M 12 yrs 7/09 T 15 2 grown kids bomb 7/05/07 H moved out 8/04/07 11/22/09 told him I quit;let's get ball rolling Mid Dec- he isn't sure he wants D End 2/2010-Starting to consider piecing