I felt very similarly when I was your age and it lasted long enough that it couldn't have been written off as simply puberty (although that is entirely possible). I'm not sure I handled it very well back then, so I hope that you do better than I did.
Do you have hobbies/activities/interests? I think having something you enjoy doing is very important and the benefits linger into those times when we're not doing them. They give us joy and pleasure and something to look forward to. Exercise and healthy diet also play key roles in emotional health. Some vitamin and mineral deficiencies are linked to depression, for example (vitamins C & D, magnesium, potassium, etc).
Then there is the question of mental/emotional health. Are you frequently stressed, upset, anxious? If so, you should consider relaxation and coping techniques. When I was your age, I would have answered no to these questions and believed it, but in hindsight I know that that was alexithymia (inability to recognize my own emotions), so if your immediate answer is "no," please take some time to consider it properly and carefully. Lack of acknowledgment or understanding of your own emotions can easily lead to depression.
I hope that you're able to discern the cause and find a healthy way of overcoming the trouble soon. Please feel free to stop by here to ask questions or vent any time.
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"A flower falls, even though we love it; and a weed grows, even though we do not love it."