KC, I know you feel ashamed, but you are not the one who needs to feel ashamed. You are the victim, he's the abuser. He's tried to frame it the other way around, because that's what abusers do- blame the victim. Signs of domestic violence- see denial.
He's the one who seriously assaulted you. You slapped him, FFS. He's brought that up with the police so he could claim mitigating circumstances for his violence not because he was genuinely terrorised.
A slap? Do you think he'd tell his mates in the pub that he called the cops on his partner because she slapped him and he's an abused man? What you did was not OK, but it was not on the same level as trying to break someone's arm. There are degrees of assault, and a jury of 12 of your peers would never say that a slap is even close to equivalent to the horrors you went through.
I'm angry at the police you saw for implying that, I suggest any future contact with the cops should go through a different police station, this lot sound awful, preferably speak to a dedicated domestic violence team.
You need to speak to Women's Aid, you have nothing to feel ashamed about, you are a victim no matter what the cops say.
You need to speak to a solicitor who can stand up for your best interests. It doesn't matter if you think you deserve punished or not, everyone even murderers, deserve to have their best interests represented by a lawyer when they get into legal difficulties. Unless you get good advice, the system can crush you. It's toweringly stacked against the individual, you are expected to get advice and are doing a serious disservice to yourself if you don't get representation.
I would also suggest that you go get that arm x-rayed, and get checked out at the hospital. He might not have broken it, but you may have small fractures and I imagine you have some killer bruises. Photograph all visible bruising. You don't have to do anything about it now, but having the evidence may be good some time down the line when you feel strong enough to press charges. If you don't document it now, you wont be able to do anything.