I sincerely hope you have rethought your position on reporting him.
The police do get frustrated when they get calls about people like your abuser, but then people like you fail to follow through, refuse to have them charged and end up going back to them, of course they do!
But that doesn't mean you should stop reporting him. So report him again.
And then decide to have him charged.
See your doctor, get the damage documented.
And get him charged.
And then make sure you NEVER go back to the fucking bastard again.
How do you explain it? You tell the truth! The worst thing you can do at this stage is make up some bollocks about tripping and hitting a cupboard door, because you're minimising the assault.
You can't bear telling anyone? Why not? YOU didn't do it, did you? HE did. HE is the one at fault, not you. HIS is the shame, the embarrassment of being a bloke who assaults women, not you.
Unfortunately too many women take this shame and embarrassment upon themselves, blaming themselves for the battering - and that's why so many of these fuckers get away with it.
Report him. Charge him. Leave him. Easy for me to say, but harder for you to do - get your sister to help you if you must, but DO IT.