The bad news is, you can't "get over this fear." As a 5 year climbing vet, let me tell you, that isn't going to happen. The best advice I can give you is the advice an Outward Bound Climbing instructor gave to me "Of course you are scared! If you say you aren't scared your first time, you're either lying or stupid." Rock climbing is a scary thing, it really is. Even an experienced climber will look down every now and then and get a tiny twing of fear.
The GOOD news is this: Once you get on the rock, and get the first 10-20 feet out of the way, you will be too busy focusing on the physical effort it takes to climb to be scared! You will, to put it simply, "forget to be scared." And when you get to that point, you will have SO much fun, you will want to do it again (But don't be surprised if the fear returns after the climb, that is normally, too).
Good luck on the 26th, and remember: Don't look down. ;-)