Dress for a formal

I got this dress because my grandmother freaked out when I put it on. I thought,”what the hell. It’s my last formal. I’ll make Grandma happy.” She wouldn’t let me try anything else on after putting this dress on, but honestly, I don’t like it. It makes me feel fat and the open back makes me self conscious. I know I’m probably being over dramatic, but I want to skip this formal all together now. If I get all dolled up, put on this dress, and still feel gross on Saturday, then I’m going to cry. Idk what to do