OMG!! I am sooooo sorry you had such a horrible experience!! It should be a fun time, anything involving your dress should be fun and make you feel like a princess!! I am soooooo sorry girl!! Can you go to someone else, or maybe a different seamstress at the bridal shop (depending on how big it is).
I had a similar situation, but i was just a bridesmaid...you should get your dress to fit perfectly on YOUR day!! And the price is outrageous!! I am having the train cut and hemmed, the entire back and sides taken in and a bustle for $150!
Take a deep breathe, i know things will turn out!