Sounds like cold feet to me. I don't mean the kind where you don't show up at your own wedding. There are different levels of cold feet. I'm talking about the kind you get when you realize things are about to get officially serious and your survival instinct says, "Run!"
It's a normal reaction. You can love someone and know they are right for you, but still have hesitations and fears about getting engaged. It's a big step; engagement makes things very real, and is a point of no return, or at least a point of "difficult to return." In other words, the window for you to get out of this relationship is getting smaller and more difficult to squeeze through. You realize this deep down, and I think your fears are manifesting themselves as a desire to break up with him, even while you know that's ridiculous.
At least I think it's ridiculous--if the urge continues for more than few weeks, you might want to rethink your commitment to this guy. Are you sure you love him? Are you sure you are perfect together? Or do you keep saying the words out loud to try to convince yourself?
If you are confident that your guy is absolutely the one for you, then I say this is little more than normal cold feet before a major commitment. To accept the fact that you're about to enter into a lifelong (ideally) relationship with one person can be daunting to anyone. It certainly was to me. But if you know you're choosing the right person, the fears pass soon enough.
Thanks for the question.
Better have it now than right when you are in front of the altar. :p