I'm not sure there was an identifiable "last straw". There was a period of about a year and a half during which ex was drinking more and more and getting weirder and weirder. One day I told him that I couldn't go on like this, and if he didn't get help in the next ninety days, I would leave. Eighty-nine days came and went and nothing changed. On day 90, I said I had found another apartment and would be moving out as soon as we had signed an agreement for shared parenting of our kid. He said, "I don't care if you leave". So I left.