Dresses run weird, so if you're a size 18 everywhere else I wouldn't worry about it too much. You usually need to size up and get it altered anyway
I know how you feel though

I remember going to dances and feeling like I was a whale in my dress. I wore a nice dress in freshman year of high school, a size 11 and I felt big then! I tried it on just two years later and I couldn't even get it on! I was mortified. I had to buy another dress for junior prom and wound up having to get a size 18 dress for prom!
Back in May 2010 I tried on the size 18 dress (and it had been altered to fit me, so it was probably a little smaller than an actual 18) and it was tight in the bust and fit fine in the waist/hips. I attribute that to lifting (and the fact that I had gone from a C to a D cup), but I was still really upset about that! I wore it to my college senior formal and I felt huge again because of that.
Now I can happily say that the size 18 dress is far too big on me to even consider taking in and while I haven't tried that size 11 dress on in a while (which in today's sizing is probably a 7/9) it fit me perfectly months ago, so I'm assuming it's too big now
You'll get there! The days they have to take in that dress you'll have a giant smile on your face!