So, I just started on this site, and I really only joined so I can hopefully get some of my questions answered. I'm a 15 year old girl with a BMR of something around 1500-1700. I have some form of exercise every day, with heavy exercise at least 2 times a week.
The point is, I have been dieting for about 3 months and have lost about 40 pounds doing so by eating around 1000-1200 calories a day. I used to go for daily 1 mile jogs, but that had my weight loss plateauing, so now I try to change it up. But everyone tells me I need to eat more and that I'm damaging my metabolism and my muscle mass and that I'm too hard on myself, but I feel fine! Is my body lying to me?

You should be eating more than that. It's not healthy to lose weight that quickly. You'll also lose muscle, and the more muscle you have the easier it is to burn fat.

I would suggest that you increase your daily calories to around 1400-1600 (and eat a healthy and balanced diet), and continue to exercise like you've been doing, and you should still be able to lose weight.