There's a very good chance that this is just cold feet...however since BOTH of you are experiencing it, some time apart may actually strengthen your relationship.
I've never been with anyone but my fiance, and for several years he was terrifed that someday I'd change my mind--that I'd want to pursue the grass is greener on the other side approach. It hasn't happened and never will...but it didn't change the way he felt. He'd had other partners and relationships before me...but since I haven't, he was afraid that I was just deeply infatuated, not in love, and not able to tell the difference...and someday, it would wear off (That's exactly what happened in a previous relationship HE had). It took a while, but he finally gets that I'm in for the long haul. I found the one for me, and I have no intention of looking elsewhere.
In your case, it sounds as though both you and your fiance are afraid you've been a little too lucky. Very few people can find their soulmate right off the bat, so you're not entirely sure that you have.
Take a break to get to know yourselves as individuals, not just as a couple. If both of you think it's a good idea, agree to see other people (not neccessarily sleep with other peple) on the off chance you'll find 'true' love with someone else. My guess is you already have, but there's no point in getting married if you're not sure of it yourselves. Seeing other people can be a slippery slope...but if you do it might make you appeciate what you have even more--because then you'll be able to see how right for each other you truly are.
You'll need to set the rules for the separation & duration together, then abide by them...and if at the end of it you decide you want to be together, you'll be confident in your choice. You won't have to wonder if you're with the right person, you'll
know. If you decide you're better off apart...at least you'll have figured it out before going through the pain of divorce. I wish you both the best of luck.