My sister was around 300 pounds during her wedding, and when I saw her for the first time in her dress she took my breath away. She looked gorgeous, and I'm positive that it's a radiant bride thing having nothing to do with weight.
Your mother sounds like she's probably very unhappy with her own life and is choosing to take it out on you and the rest of the world. I've worked at a restaurant for 5 years, and I encounter so many ungrateful and downright rude customers. Initially, these encounters would bring me down and really throw me off, but I've learned to let them serve as motivation to become an even kinder person. Let your mom's harsh words do the same. Let them motivate you to be the best friend, wife, mother (?), even stranger that you can, so that everybody can count on you to brighten their day!
Marry the heck out of your fiance, and surround yourself with people who lift your spirits rather than try and bring you down!