My ex only hit me once - I left him after that though. When I looked back over the relationship, there was another incidence of violence that I had normalised, and lots of controlling behaviour.
However I did speak to his ex after the violence to warn her (and he corroborated what she said), and she told me that he had been violent to her, and that the behaviour had escalated his behaviour with her - getting increasingly violent, right up to pushing her down the stairs, and putting his hands around her neck. My view is that once it's happened once, and you've let it go, there's a boundary that's been broken. And they know that. It becomes easier to justify extreme behaviour.
(interestingly when he hit me, he was shouting out her name - it was terrifying. The hate . I still have flashbacks to that night).
But more terrifying was that she chose to go back to him, with a child, and risk her life and that of the child. There is clear evidence that domestic violence escalates, and putting hands round neck is a massive risk factor. I do not know why anyone would go back if there is even a tiny risk of violence, but with a child? It's selfish to the extreme I think.
I don't know if you've got children OP. But it was that fear that stopped me going back to him. He pushed me onto a concrete step, and I fell hard. I was very bruised. But it could have been worse.
I have also read the Gift of Fear and picked up on that quote (quoted by a PP) - if you are hit once you are a victim, twice and you are a volunteer.
Good luck Op. My experience was that he just worried about himself. Begging, and pleading, but ultimately just worrying about his own reputation. He went on to accuse me of lying. I wish I was.