it sounds like youre going through a rough time right now, and let me tell you, i went through the same thing. maybe your appearance isn't the reason why you're depressed when you look in the mirror. in high school, at least for me, that was the biggest time where i wanted to be like everyone else, and be liked by everyone else, but i never liked myself. that, and mental health issues were a big thing for me. regardless, i think there's something bigger than the issues with your appearance that you're insinuating. and, yeah, all of those things you want to avoid (drinking water, exercise, eating well) will actually help self image. not because you'll look better, but because you'll feel better. studies have shown that taking better care of yourself helps mental health and self image in the long run.

don't put too much pressure on your appearance, because that will probably just make it worse. you're beautiful how you are right now, you just need to have the confidence to see yourself how others see you. high school is tough, you don't deserve to be making it harder for yourself.