Well, I'd start with figuring out exactly why it is that you cannot give more than one person your all. (:
A lot of people feel very specific ways about things but when it comes down to it they can't really explain why, logically. I'm a very logical person and like to base my beliefs on logic as best as I can, so anyway that's where I would start. Get yourself figured out. Once you've got that down things should be a lot easier for you.
You'll either figure out that you really do want monogamy, accept that this man is not into that and move on, orrrrr you may open up more to the idea of non-monogamy and who knows what will happen from there, (:
Last thing, don't be afraid, whatever you do. Fear holds us back, so, so, SO much! Love's the way to go (: