How do I feel okay again?

I'm 17, so I really shouldn't be complaining. I know it's probably just the "high school sucks for everyone, and it'll get better after you graduate" classic. But I just can't anymore. All of my friends are doing even worse than I am, and my brother's doing odd things that I have no way of finding out what it is. I'm drowning in band and AP classes.

Every day feels dull now. If I'm not around another person, it feels like nothing has any purpose. I can't bring myself to do anything but be miserable and anxious. I can't tell if everything around me objectively sucks right now or if I'm just really depressed. I don't know what to do.