I feel like I wrote this post, that's how similar my situation is to yours.
At the beginning of my relationship, he acted sweet and caring, he wasn't drinking a lot. He was doing triathlons and marathons. After 6 months, he quit the athletic activities and started drinking heavily. When I would talk to his friends when I was concerned about his drinking, they would say "don't worry, he is half as bad as he was a year or 2 ago!" I figured it was just a phase, but it went on like this for an entire year.
6 weeks ago, he broke up with me completely out of the blue. Prior to this, I had a back injury that lasted for 3 months, and he was completely unsupportive. He kept telling me I needed to snap out of it and not be upset. He would come home so wasted to the point of not being able to speak or walk. I felt like it was my fault, because he would always blame it on. When he dumped me, he told me that I caused his drinking problem and that i was "a weak-minded dependent leech" who he could "control" because he knew he could do better than me.
I was devastated, and still am a little, but I honestly can say that I am in the same boat as you. It is not our fault, it is theirs, and they will struggle with this addiction their whole lives, even if they are "cured" by therapy. Good luck to you