'How could I have let it get that bad??
Why couldn't I have made DH happy then none of this would have happened.'
Ah now, come on Fairycakes -- he didn't hit you because you lost control of him. He didn't hit you because he wasn't happy. He didn't hit you because he was losing his temper.
He hit you because he was using his 'temper'.
This may seem very blunt, but you are not special to him. If you were, then he would not have hit you, end of. You are not special enough to him to make him 'happy' or to prevent him hitting you again.
You are lovely, you are young, you are a caring mother but he doesn't want any of that. He wants a punchbag. That is what he saw in you. A punchbag crossed with some magic being who could give him 150% of their attention all the time, or so he fondly imagined. Someone to adore him warts and all and never have any needs or opinions of her own. Then along came the baby and he couldn't cope with the reality that you are not there 150% for him all the time. He does not care about you at all. He cares only about what you can do for him, and what you can do for him is not at all what you think you can offer. You want to give love he doesn't want that. He wants someone to vent his frustrations, insecurities, immaturity and self-loathing onto, to project all his own failings onto, and to eventually destroy.
'I just want my family back and for all this to be over, it's making me ill.'
-- Please grieve the dream you had of a nice family life with this man. Go through the sadness of that grief. It's like losing a relative you were very fond of. If you won't go through the sadness of grief for the marriage, you may well go through the grief for the son SS will remove, or the grief of seeing him turn out like his dad, or the grief of turning into a shell of your former self, staying home instead of going out with bruises showing.