Never been "slapped" or suffered physical abuse. So can't address the leaving the first time.
But re identifying red flags, not putting up with things.... I worked as a professional in an area directly related to domestic abuse. I knew this stuff well.
But he didn't ever stop me seeing friends. I just lost touch. Didn't see that he caused a row on my way out EVERY single time. My few nights out a year always started in the toilets having a cry then covering up my red eyes.
He didn't like my decor taste, I didn't mind his. So of course it made sense for the house to be entirely to his taste.
How ridiculous to be scared when he shouted at me or DC. He laughed at me, then admitted sometimes to being angry. But like he said much easier to take ADs for my "overreaction" than for him to get help for his anger.
When i was too tired for sex post children and he repeatedly asked me to do things I didn't want to i could have walked away, my fault I gave in. When I moved his hands and he moved them back he was being affectionate. When i said no to him taking my clothes off and he got fed up and did it anyway he was just helping me find my mojo.
And so on.
One day you are wondering the streets too scared to go home, too scared to ask for help.
Was it my mental health, was it a relationship gone stale, was it abuse.
It's not obvious when things go wrong.
That I could have helped myself, that any self respecting woman would have done things differently is very difficult to deal with.