You are 15 and you haven't identified any major issues besides the symptoms themselves so I'm going to assume it's the result of chemical changes in your body due to adolescence. The feelings will pass as long as you don't feed into them. Don't sit around all day feeling sorry for yourself and let it metastasize into major depression. If you want to do something, do it, it may require more effort than it used to, but put aside the time to try anyway.
Try and enjoy the next few years of your life to the best of your ability, because when they are gone they are gone forever, and everything just gets harder and harder from there on out lol
NOTE: If you opt to tell an adult, which is a good idea, tell your parents or your siblings or your aunt or grandparents or something. Don't tell anyone at school.