My boyfriend slapped me yesterday and I cant stop thinking about it; help?

I'm not sure why I've posted this: for advice, or maybe just as a way to let my feelings out. I've been living with my boyfriend for the past year as we're both second years at the same uni - we've been together for three years. As soon as I moved in, though, he's started behaving so erratically. He makes mean comments concerning my weight, physical appearance in general and my past in mental illness. He says he doesn't like my friends, the way I dress when heading out - he even admitted that he didn't really like my siblings much. Yesterday, I saw him getting ready to go out himself, and I asked him where to, since he told me that I wasn't allowed out at all and I thought it was hypocritical. He seemed blunt at first, but soon worked himself into a fit, and when I stepped a little closer to see if he was alright, he lashed out and slapped me hard. I'm so shocked by this. I've always seen these things, and heard of these things happening in the media, or always to someone else - but I never imagined it would honestly happen to me. He went out anyway, and came back apologising. I forgave him, but now I think I'm starting to regret it. I haven't told anyone, and whenever I start to mention it to him, he tells me to be quiet because it's "in the past". But, regardless, I can't stop thinking about it, and I really, really don't know where to go from here.

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