Why can't I leave him alone?

About 3 weeks ago me and my ex split up. We have 3 children and one on the way. We split up as I'v ejust become disenchanted in the relationship after going through so much with him.. he has lied to me, cheated on me and become physically abusive towards me and very bad tempered towards our kids. 3 weeks ago he knocked me clean out, outside of our house after I shoved him back because he had shoved me in the stomach twice (I'm pregnant) and I just saw red. Although he's now going around saying I hit him first.. which I didn't.. and seems to actually believe this! My problem is this: I know he is no good for me, I know what he has done is inexcusable as well as all the cheating etc but I still can't seem to leave him alone. I was fine for the first couple of weeks but I just feel awful now and he is just being so indifferent towards me. He has barely apologised and hasn't asked how I am or anything despite knowing I've been really depressed. If the situation was reversed I'd be mortified about the whole thing and constantly apologising and trying to make things right. He's not bothered. He says he's sick of the way I tell him he's got to change etc. I know this is all totally ridiculous so why can't I just leave him alone? I don't even want to be with him! It's just when I get a bit lonely and the fact he's being so horrible towards me after how nicely I've treated him over the years and that we have kids is so hard to come to terms to. How can anybody be that uncaring? I just don't understand it.. and why is it I'm the one getting upset thinking how he won't get to see the kids much and things like that when he just doesn't seem to care.