To me it sounds like you're generally 16. You're life is 'complicated' and you have 'big problems', but that's about all you get into, just this vague notion that you have a lot to deal with.
I understand, when we're 16, we feel like all our problems are big problems and that no one could possibly understand how complicated our vaguely complicated life is.
But honestly, unless you can vocalize all those problems in a concrete way besides the few abstract examples of not being able to concentrate, or feeling sad or having to deal with new things, they really aren't there. No person on the planet doesn't understand what it's like to not be able to concentrate sometimes. No person on the planet never feels sad. And no person on the planet has never had to deal with new things. These are all normal and mundane things, and not big or complicated.
Best to realize that you're 16 and to stop stressing, because eventually things will get complicated and big, and then you'll wish you had your 16 year old life again.
Cheer up! Cheer up! You will have to go through it. I have experienced before. Of course we know, study is very important for the your future. when I lose my mind, I find a hobby, exercise, go to church,date, volunteer, hang out with friends, but I can't feel good,finally I'm play guitar,it make me surprisingly good. May be you should will find a hobby.
It sounds as though you could be struggling with clinical depression, although I would talk to a doctor about it first, trust me, I had clinical depression once, and i had to take meds for it, and it made me feel "normal" again