This sounds like a pretty shit situation, I cant lie...

I'm no psychologist or counselor or anything, but I think you need to sit down with your mum, and calmly explain to her that you have issues, and your social struggles to her. However, try to see tings from her perspective, and understand that she may find it hard as a former "popular kid" to understand

In addition, it may also be helpful to put things into perspective - It's a high school prom... I know things are different over here in England, (my school nearly cancelled the prom because ticket sales were so low), but it's effectively a just a glorified year 6 disco, with the teachers watching over your shoulder. What I'm really trying to say here is, you need to get to the root of why your mum takes prom so seriously: is it because you live in a small town, is it because she's a high ranking member of the PTA, or is it just because of cultural pressures in America, (I'm guessing that your american because of the way you write/ phrase things). Really try to calmly talk to her, and drill down to the root cause of her frustration. Once you've done this, I would try to change the way she thinks about prom/ convince her that it shouldn't really be such a big deal. Just talk calmly.

If your mum fails to understand your personal challenges, which I, and thousands of others on this subreddit can relate too, then you may need to seek outside support, as this genuinely sounds like a pretty damaging experience, especially for someone who maybe struggles socially. Just know that to most people, not going to prom/ going alone doesn't make you weird.

P.S. Sorry if this doesn't flow to well, sorry, but it's 2:30 AM here and I'm really fucking tired.

P.S.S. I'm happy to keep talking to you on here/ on another platform if you want.

P.S.S.S. My mum is a counselor, and I can ask her for some advice on how to talk to your mum if you want.