Believe in yourself and your future.
Understand that most all of us went through a stage, at different times in our lives, where everything went all to heck emotionally and we felt like "what's the use?"
Know that it will pass. That life is a rollercoaster, (to use an old cliche), and the ups far outweigh the downs.
About now though: Get involved in something. Start doing something creative if you can't get out and about. Start writing, reading, exercising, painting...anything to get your mind to concentrate on something other than the here and now.
When you start feeling like you want to cry, decide not to cry. Tell yourself that you are in control and practice acting happy. It seems too easy to work, but once you start conditioning yourself to act happy, pretty soon you are.
Also, you're 16 years-of-age. That is a very hormonal time of life where your body goes through a lot of chemical changes. Yes, physically you've already had a lot of changes, but the body is still growing and changing. Identifying it as a chemical reaction may help you understand and eliminate the side effects.