Daichi,
Sorry to hear how you're feeling. About once or twice a year, I get hit by something similar. Mine usually last about two weeks, but this year mine went on for about two months. My Dr. is unsure of what I have but guesses that it's a virus; possibly low grade mono (chronic fatigue). The problem with most Drs. that aren't familiar with endurance athletes is that because we're not terribly sick during these bouts, they feel that were fine. They don't understand how much of a stress biking is on the system. I'm not saying that you have what I have, though. My advice is that you're 16 and have a many years of riding ahead so take the time to get well. If you do ride, don't go anywhere near the anaerobic level. It would probably be best to stay in the recovery zone. Listen to your body. On days that you're feeling tired or unmotivated, don't ride. Trying to ride through your fatigue will do more harm than good. Good luck. Take care of yourself. Hope you get better soon.