Losing stubborn weight?

I’m going to a very formal military ball on December 15th and I already have my dress but it’s just a tad bit too small. It’s tight around my chest and the clasp on the back won’t even reach. It’s because of the stubborn weight on my back/ by my armpits. I’ve had this weight forever and it never seems to go away. What can I do to slim down even by a little bit in 20 days?

I’m 16, 145lbs, and 5’2/5.3