Disclaimer- I didn't know my Xh was cheating/had cheated on me when I left him.
I realized D was what I needed when I stopped giving a shit about him. About the same time I got pregnant with our son (which was an accident- switched from the pill to the Nuvaring and... anyhoo). I had never wanted kids with him and now one was coming. I didn't want to raise my son in a household so full of tension and anger and emotional abuse.
I stopped caring. All the lies (over money, drugs, his whereabouts, etc) for 3 years just piled up. All the fights, name calling, staying out well past when he said he'd be home and turning off his phone, walking in the next day like he'd done nothing wrong and then getting furious with me for being upset. It all just chipped away, until there was nothing left.
One night, he went out with friends, was supposed to be home when the bar let out (2 a.m.). He didn't come home until 10 a.m. And I just didn't give a shit. That day I started saving $ for a divorce and I basically just let him do whatever. He was the happiest he'd ever been. He started telling everyone I was the coolest wife ever. Little did he know.
He was completely blindsided when he came home and found all his shit on the front porch. Later, I learned he found great comfort in his girlfriend.