Scout, you are wise beyond your years and experience. So much of what you wrote is something I could have written. I applaud you for standing up for yourself and your son and recognizing that you deserve so much more than this. I wish I had the strength and the courage to do it 10 years ago. But in truth, like you, I see things even before then that showed me I deserved better.
Whenever I feel wobbly, I do remember that. OD is the one who is missing out. OD is the one who will have the life-long regret. As it concerns me and the children, I will always have a bit of a broken heart, but I'm guessing that is nothing by comparison. At least I don't have to live with the fact that I broke three of them for my own selfish and ultimately pointless endeavors.
Leaving this cleanly with as much compassion as you can muster will make the next 17 years a lot more pleasant for you and your son.