I had quite a few "Last Straws"
1. Last straw for "I don't want DS to grow up like this" was when I found a picture on my digital camera that DS took when he was 2 or 3 of his Dad. The picture was actually of XFIL's puppy, who we were babysitting, standing in the bathroom. AXH was in the background, 1/2 fallen into the tub, shower curtain ripped down with him, looking like he should have been passed out hours before.
2. I knew I had to leave him when one night I realized that I had been sitting locked in the bathroom on the edge of the tub for hours trying to figure out the best way to end my life and fast on the heels of that revelation was that if I actually followed through, DS would be left all alone with AXH.
3. The moment I realized that I didn't have to put my life, and DS's life, on hold to wait for AXH to get his act together was when he got back from rehab and said everyone told him he was sick, but he didn't see it and that it was always the hard stuff, not the beer. He still couldn't see he had a problem. His statement showed he was already getting ready to start up again. I was scared and didn't want to be stuck with that any more. And then it clicked that I didn't have to be.
As much as he and his then-GF kept saying I only filed for divorce because he went back to her, their relationship had nothing to do with my decision to file for divorce. It was all related to the revelation in #3.