d have to lose 16 lbs/month, which is almost twice as much as the recommended safe weight loss.
At 17 your body isn't fully developed and losing that much weight, that quickly, could cause you a lot of problems later on, plus it will be impossible to maintain the loss, as you'd have to use measures not intended for long term use.
Maybe you could look at a different dress or get it in a different size? It's good that you want to lose weight, but you have to do it with reason.