Katie I so understand what you are saying. I have turned my back on my ex. He was my best friend. We shared a wonderful life together. I asked him to leave 6 months ago and stayed emmeshed for so many different reasons. We have a child together so I had to see him regularly.
When he went to treatment he got "better" but still continued in his addictive behaviors. Then relapse. At first I was ok with it. Then FEAR set in and I started going nuts.
Breaking up with, leaving, divorcing someone you love who is an addict is the hardest thing to do. IMHO. What is stuck in my head is that I didnt leave him because we didnt get along, he cheated, I found someone else, we grew apart, or any other reason. I left because of DRUGS. To me that problem is fixable. You know. So then I have a hard time leaving the relationship in my mind because its fixable.
So then you are stuck. I am moving forward a little each day. Sometimes I condemn myself because after all its just a break up. But its not in my mind.
The hardest thing I have ever had to do was to leave someone that you love and turn your back on them because your own life is unmangeable. And I left an abusive marriage years ago.
Just keep moving forward. I know its hard. but its possible.