How to overcome being burned out/burnout?

I'm a junior in high school with the top GPA in my cl of 1100 students. However, these past couple months, especially the last one, I have been slowing down a ton. It's difficult for me to concentrate on my work because I feel overwhelmed, and I end up trying to find methods to avoid it (napping, eating, checking my email, etc). I end up pushing my work until late at night and getting to bed at 2:30-4:00 pm. It's getting really ridiculous because I know I could get it done if I start it earlier, and I just feel exhausted and can't seem to get myself to start it earlier. I'm feeling incredibly depressed, lacking self confidence, and feeling a ton of pressure; this is my 2nd semester of junior year, and I'm supposed to get all A's to go to ivy leagues or get scholarships to less prestigious schools. There are 3 six-week periods that factor into the semester grade that colleges see, and in the first period I just got 3 B+'s, staying up way late. HELP?