During my teen years, I felt this way too. It seems like around my early teens, I went through a little bit of depression. I think I honestly felt a little lonely since I moved often and didn't have a tight knit of good friends. I felt a little left out and out of place. I was a good student and did well in school, but I was self conscious and hard on myself, and didn't always feel happy. As the years went on, I got better, but would occasionally feel worse again. It's almost like a cycle. I was struggling one year in college and decided to talk to a counselor and went on antidepressants. They didn't really help me and I decided to go off of them, but this past year I recently tried an anti-anxiety medication since I was always anxious and panicked feeling. They have helped me a lot and helped my mood too. I get headaches from stress, and now I get them much less often. Being anxious makes me feel tired all the time too!
Do you generally feel pretty relaxed, or do you tend to worry more? If you tend to worry more (like me), it can really take a toll on your energy!
If it's not this, it's common to have your moods change like this around your age.... your hormones are changing and this can very well happen! Is there anyone close to you that you can about it? I talked to my mom about it and found out that she went through (and still does) the same thing. It can be comforting to be able to relate to someone else who feels that way.
Do you feel happy? Sometimes if you feel like something is missing in your life, no matter how well everything else is going, you will feel a little bit empty and sad at times. I try to make the best of everything and be positive as much as I can and not let the negative things bother me so much. I remember that some things are out of my control and try to move past them!
Definitely let us know if any of this sounds like anything you deal with. If not, there are other things too! Just more ideas to think about

Hope we can help out!