I am unsure of just how form controlling they are. I own a spanx for a health reason and not for vanity. I needed something to wear while I was waitressing a few years back that would provide me with some trunk support and not be bulky underneath my clothes. My mother picked me up an underboob spanx that clipped under my vag. A doctor of mine thew out the idea as a possible suggestion. It did give support for sure, but it also cut off my butt cheeks at that time, leaving marks and making it look like I had a double butt.
There are all kinds of spanks, and I have seen them really smooth people out. I would look up reviews online to be sure, and also if you are looking to try and diet a bit for the wedding, a fast way that has been known to be effective, is cutting out carbs as much as you can. You wont have to stick to it if you dont want to after the wedding, but it could help you to fit in that dress. Do whatever makes you feel comfortable and healthy. Best of luck.