How do I break away from this guy?

This is actually a follow on from this situation, if you want a little background.

I've been dating a guy for about two months, although fairly recently I ended things for about two weeks...I tried to get out, but I found I just regretted ending it and really couldn't get him off of my mind, I tried to look for new dates to take my mind off of him but I was comparing everyone to him and stressing myself out thinking about him all the time. He contacted me and we're now back together.

He cleared up a lot of things, but it's not really much better - I thought he was using me for sex but he's not, however he's looking to date/have sex with multiple other women - I sort of knew this was the case, this is why I ended it and it should be a deal breaker for me.

So he doesn't believe in relationships or love, he's into dating multiple women at once, also he's into things sexually including threesomes - there's nothing to make me feel he's not being genuine in having a different view of relationships/love (not just a run of the mill jerk). He claims to like me most of all and not to have had sex with anyone else yet, but I can't know if it's the truth or how it'll be when it changes.

I'm monogamous, I believe in love, I like long-term relationships with security of trust and exclusivity, my value is tied into a guy being totally into only me and me alone. Even the insecurity of starting a new relaitonship or dating people turns me into a basket case.

This guy has me so totally intoxicated with him that I can't seem to break away, I already have feelings for him and damn it if they're not going to grow if I stick around...he's great on so many levels: attractive, great in bed and could get better, very intimate, very interesting, socially aware, arty, I feel safe/comfortable with him like I've never before with anyone else, and I feel happy/content when I'm with him.

BUT...I'm anxious thinking about him with other people or thinking how easily replaceable I'd be, I don't get enough attention from him, and I'm worried about how I'd keep him interested sexually...I'm worried I might try to push myself to be okay with things I'm not okay with, or if he rejects me I'll blame myself. Ultimately I'm going to have my heart broken and I'll feel like a loser for getting into this situation, I know better...I've known from the start this wasn't a good idea...I know I should have a little more self-respect and it's healthier to get away now.

I've never felt this way, never gone back once I've left.
I can't break away from him.
I don't want to break away from him.

I'm once again trying to convince myselt that I can be casual with this guy, just have a little fun until ready to move on...enjoy the potentially great sex and experiment a little, enjoy time with an interesting man...but I know this will hurt like hell when he rejects me.