Nice to meet you. I am a 19-year-old , so around your age. Just as Karen said, the time right after puberty is the time the great questions of the world can kick in and that can take its toll. Though in your case it could be rather different, so you feel free to tell more about yourself and what you are struggling with.
Talking helps. I am dead serious about this, it really helps. Whether it is to friend you trust, your family, or a counselor. I too have friends and a really supportive family, but not talking made it a lot worse. So please, find someone to talk to. Of course, you can talk to us, but the more the better.
Also, it could be a hormonal imbalance. In case you are unaware, we have certain hormones that trigger a feeling of happiness. In some individuals this system isn't working properly, so that could be the cause of your sad feelings as well. A doctor should be able to help you with that. In fact, seeing a doctor is a good start in any way, as he can show you where the support groups are, or advise you what to do.
You say you feel lonely. It is a common symptom of depression. Are you someone who ponders over that again and again? If so, I might be able to help you. But the lonely feeling would probably not go away without the help of others, as darkness won't go away without light. Let your friends be the light. It is hard to tell if someone is lonely, so the best way to let others know is to tell them. Than they will be able to help.
And Forever, remember that the most important isn't school, isn't money, isn't status. All of that we want because we think it will make us happy. I do think getting proper education will contribute towards your happiness, so if it is possible, study the best you can. However, if it makes you sad, don't do it. If something makes you feel real bad, try to find different ways around it. Hurting yourself is never the answer.
Take care my friend,
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