Bridesmaid dress meltdown

So I got asked to be a Jr bridesmaid for my aunts wedding. Obviously I said yes. The wedding is in 2 months so we’re in a rush to get me a dress and she wants the dress to be as close as possible to the bridesmaids dresses. So we went to the dress shop and somehow magically, on the discount rack was a dress that was incredibly similar and almost the exact same color as the other dresses. And it was only $25. Obviously my aunt was like that’s the dress. So I went and tried it on and it didn’t zip. It went up halfway. I tried on like 12 other dresses but no one would let go of that dress. They made me try it back on and they were doing everything they could to get it to zip. Finally they went and grabbed the manager and she was like, our seamstress might be able to get it to work. They pulled out every way physically possible to get that dress to possibly fit me. Convinced my aunt it might be somehow possible, and then we bought the dress and now we’re done. I’ve been crying for an hour. I feel like shit because I can’t fit the dress. Everything about that dress is perfect, except for me. Because im too big to fit it. I have an ED and I’ve literally never felt worse about myself. I feel like it’s my fault that I can’t fit the dress. I can’t stop thinking that like, maybe if I just don’t eat for a while I’ll be able to fit the dress. I don’t know what to do. Like why can’t I fit the damn dress. Why do we need to get a magician of a seamstress to somehow magically make that dress fit me. I feel terrible.