from everything i've read losing a lb a week in feather weights is very rare. so you may need to adjust your expectations. just follow your plan, and see what happens. your body will need time to adjust.
If you are missing your periods you should really speak with a doctor. At 16, your body is still building bone mass. Good nutrition and menstruation are key parts in this process for women! I'm glad you're reconsidering your goal weight.
