Lots of good advice here, but I too, am I second time bride, and I had doubts the first time and in retrospect, I should not have gone through with it. The closer it gets the harder it is to call it off, but YOU are the most important person in YOUR life. Trust me, it's much more painless and less costly to do it now before you become the Mrs. than it is to do it later down the road. For me it was 16 years and 2 kids later and it was very hard. There are no guarantees in life, that's true, but if you are really unsure before you walk down the aisle, that's not a good way to start.
I hope you can find some time alone to try to think it all through. You deserve the best....and you have lots of support here regardless of which route you choose! Good luck!