Constant headache, lack of sleep, loss of appetite, weight gain?

These are symptoms. of depression, but could be caused by a myriad of physical conditions, which need to be addressed and eliminated first. Start with a visit to your MD, then after that, follow his/her recommendations for further care. You're 16. This is an age of great turmoil--socially, academically, personally. You might need some support to get through it. Hang in there--all adults have done this.