I also wanted to say I hope you are feeling better about things. Like others here, I know that, no matter how much you weigh or what dress you wear, you will look beautiful on your wedding day.
I actually almost didn't fit into my wedding dress, because I was trying NOT to lose weight. My doctor actually told me to lose and I told him the dress was already at the seamstress (I bought my maid of honor's dress from her and had it altered down - and it looked better on me -
) and that I couldn't afford more alterations. So I didn't watch what I ate and ended up feeling like I couldn't breathe all night! It fit, but BOY was it tight. On the up-side, most wedding dresses are made to be form-fitting.
Now for totally practical advice. If you already have the dress and it's too small, speak to a seamstress NOW and see what can be done. I've seen some lovely dresses that have laces in back instead of buttons or a zipper and they are very forgiving in fit. Perhaps the seamstress could replace the closure with lacing and a modesty panel if the style of the dress allows it?
If you are really afraid it is not going to fit, period, then you need to look NOW for a replacement. I don't mean to add more stress to you - but a few weeks may not make enough of a difference if it's a size or two too small and you
don't want to try it on a week before the wedding and realize
then that you won't make it to the right size in time. At the least, get a backup dress waiting in the wings. There are plus sized catalogues that sell wedding dresses and some wedding shops may have some in stock, but if you have to order it will take some time. Depending on how big a city you live in, you might consider renting a dress. Perhaps you can borrow from a friend/relative who was your size when they married? There are alternatives and while it may not be your dream dress, you can find a dress to wear.
Every woman wants her wedding to be perfect. After
two weddings, I can tell you that while the wedding is nice, it's so much more important to BE married than it is to GET married!
Best wishes on your future marriage and I wish you much happiness not just on your wedding day, but for all the days to come!