Your body is beautiful to the people who love you.
Try taking your daughter or a good friend with you and actually hit the high street rather than relying on the internet. You can try things on and have a giggle while you're at it. Make a day of it, have lunch and mooch around the shops.
It may just be an accessory to go with your current outfit that will make it sing for you. or you could realise that actually, the outfit you already have was really nice all along.
I think a prom dress looks nice on a pear shape. Fitted in the body and then flaring out from the waist. Knee length is my preference but about an inch above the knee for you because I'm taller.
This should be an exciting day. Please don't let your perceptions of your body spoil it. The people who matter and who love you think you are a stunner!