My voice sounds HORRIBLE and I have a talent show tomorrow!!?? HELP?

So according to other people I'm a very good singer so I auditioned for a talent show at my school and I'm going to sing Back to December by Taylor Swift. Well today is the day before the talent show, and of course my voice has to sound awful. I sound all nasally and horrible, and my voice is constantly cracking. I'm pretty pissed off right now because I can't even f****** breathe right when I sing today, and I'm constantly cracking on random notes that I can usually sing. I'm going to be so freaking embarrassed if I ruin the song in front of all those people. I've tried everything...drinking tons of water, doing push ups, sit ups, drinking hot liquids, breathing techniques...and absolutely nothing helps. WHAT DO I DO???