I honestly can't say what exactly it was that happened the day I decided to quit, I just knew it needed to happen. It was a culmination of several of the things already listed here ( denial, selfishness, health issues, etc ).
That's one of the things I continue to struggle with when trying to help others find a way to do it. I wish I could tell them why that one day was the day for me but I cannot...I just knew it had to end right there and then. I didn't have any sort of epiphany or vision either, I just knew it was the day to quit for good.