We're glad you decided to talk to someone about these bad feelings you're experiencing. Unfortunately, we can't tell you exactly what it is you're suffering from, but we can tell you that your symptoms sound really frustrating and maybe even dangerous. You said you're worried you "will do damage." What kind of damage? Hurting yourself? Killing yourself? Hurting or killing someone else? Whatever sort of damage it is, damage is never good.
Considering what you've told us, we'd recommend talking to a professional about what you're feeling. You can do this by just talking to your regular medical doctor, or you can ask your mother or father to make you an appointment with a counselor. We know you said you'd rather no one know, but it really might be worth it to tell your parents that you're feeling a lot of low moods lately and you'd like to check with someone about them. That's a wise thing to do, so hopefully your parents will help.
In the meantime, try to find support in the form of an adult in your community or family. For instance, you could pop in and talk to the school guidance counselor every week or so, or you could call a relative and talk a bit. Think of neighbors, teachers, religious people, coaches, etc. There are many, many adults out there who'd be happy to listen and help you, but you have to find them. Also, keep busy! When you're feeling down, one of the worst things you can do is nothing. Find something that really engages you or relaxes you. Don't just lie around feeling bad, because that prolongs it.