Dress shopping...

I have to go dress shopping with my mother tonight, and I don't know what to do. I've tried everything I could, but there's nothing I can do to get out of it. I asked to just wear pants to the event, but the paper said that "girls have to wear a dress or skirt," and I'm not out so I fall subject to that rule as well. I put it off for as long as I can, but the event is tomorrow, and I still don't have anything to wear. I really don't want to do this because the last time I had to try on a dress in a changing room in a store, I had a huge panic attack, and I didn't even know I was trans back then. I've been so nervous all day that I've made myself sick, and I've been on the verge of an anxiety attack all day long. I'm so scared that I'll have one in the store that I'm freaking out about freaking out. I guess I just need support? Maybe if you guys have any calming exercises that you do, that would be nice too? Please.