Horrible first dress fitting appt

A little back story.
When I purchased my dress I went to a David's bridal in Manhattan.
They squeezed me into a size 18. And suggested I get into a 20. I mentioned that I started working out and plan on shedding some weight for the wedding. They said "okay that's good but they can always take it in a size" so me being naive I just went with it.
Well fast foward 3 months later, today I had my dress fitting in the David's bridal in queens. and they had to take it in so much but i didn't give me any shape, I looked like a rectangle. And I'm a plus size girl and very self conscious about my body, plus I added sleeves and a ugly bustle that I didn't not like so the total of the alterstujns came out to 700 and my dress cost a little over 800.
I had a bridezilla moment and started crying and staring that I felt that they tricked me into getting a size 20 so I would have to do all these alterations and pay all this money.
I told them I dont mind paying for a service but having to pay almost the same price for my dress bc it's too big bc they insisted on a certain size was unfair.
So they now said they will exchange my dress from a 20 to a 18. And expected delivery date is June 17. My wedding is July 3.
I'm super nervous, upset and a bunch of negative emotions.
Now I'm nervous that I'll have to be charged extra for rush free and just feel that I was completely uneducated on bridal shopping.
I wish I knew better.