I had the same issue. My wedding was in September and I started trying on dresses in February. It was a horrible experience. I had gained weight when my mother was sick and I hated myself in every dress that I tried on. I was a size twelve but had to try on dresses that were a 16! OMG, I stopped shopping for a dress. When I lost 10 lbs, I went back out and found my dream dress, size 8, to try on. I got it over my butt, but could not zip it. I was so worried I would not lose anymore weight so I ordered it in a size 14. When the dress arrived, 4 months later, it was to big!
My point is, don't go shopping for a dress to soon. You don't mention when your wedding is, but I would allow 6 months before your wedding to start shopping for a dress. And like the others mentioned, try some of the larger bridal shops. They will have more of a size selection. Don't be upset and don't look at the size on the tag. Look at yourself in the mirror and marvel at how beautiful you will be on your wedding day.