Daisy,
I hear your discomfort coming through loud and clear. It must hurt like hell to care for someone who is with someone else.
I, too, separated from my spouse. Then he found another addict to live with. That new relationship of his lasted about two years until she got put in jail for drugs. (How embarrassing! She was taken out of his residence in handcuffs!)
I met him when I was 19. I used for the first seven years into our marriage. My spouse was a perfect co-dependent. He needed me more than I needed him. After I got clean and sober, he got more and more angry with me for getting healthy. I stayed for three years, even though they told me I should give the marriage at least TWO years after getting sober and clean! Like I said, after I left, he found someone else, another addict, except this time, she was using. Guess he needed someone he could pity, take care of, and look down at.
That ain't me. I am healthy now. Too bad he couldn't get healthy "with" me! I gave him plenty of chances.
It's not quite the same as the situation you described, but it is similiar. Thought it might help to see the other side........
But, then again, maybe not. -tabfan