The moment I decided to leave was when I realised that I didn't have another chance left in me. However, the build up to that lightbulb moment was multi-faceted.
Bottom line - I could no longer sacrifice myself at the altar of someone elses issues. I had (and have) enough of my own to deal with and I was finding it hard to stay on track whilst being actively thwarted. Those last few months between starting counselling/attending meetings and leaving almost broke me emotionally.
Life is far from perfect nowadays, however it is immeasureably better than the alternative.