Narcissistic Mother

I'm 17 days from my wedding and my mother an I have had a terrible relationship for as long as I can remember. We have 19 years between us and I've always felt like more of a mother to her then she was to me. But I think after my wedding I just have to let her go forever. The whole wedding process has been misery. From "you're so lucky to have a bridal shower, I never had one" to "I know you're paying for this wedding all by yourself but it's not just your day it's MY DAY too". Any way last night I snapped. My mother purchased an $800 PROM gown to wear to my october winery wedding. SHE CLAIMS is a MOB gown but I researched the designer and realized they only produce prom dresses not MOB dresses. It's embarrassing. She's trying to out shine me. She's 43 and trying to act 18 like the model in the picture of this Rhinestoned gown with a long train and keyhole back that's bright blue. I told her I didn't like it and she said " I don't need your permission, it's my day too" PLEASE HELP ME.