Why is my BRAIN so slow?

exercising and not staying by the tv all day or the internet this will help im 14 as well and got past that

Keep sleeping enough - this'll be necesary for a good concentration. Also, try finding other activities in your life in which you can concentrate - reading a book for example, or playing a game.

I have personal experience of being very distracted, considered a bit dumb and lazy. In the end it turned out whatever i was studying was just too easy (not trying to boast here), and as soon as i jumped a year, everything immediately went better. Its just... too easy to pay much attention to. The opposite is also possible - you just don't get what they're trying to teach you, and nobody explains it better, so you get distracted because you can't do anything, because you don't know how. Consider if either is a possibility for you - try finding a textbook for the next year and try learning the first and second chapter (first chapter is regularly just refreshing your memory). If you have no problems whatsoever doing that, you might want to consider finding something more challenges at school, perhaps trying to jump a year like i did.

If not, look around the time it started happening. Is there something in your life that has changed dramatically which could have made you distracted? What kind of things do you think about when you're being distracted from your work? Maybe you can find the answer there.

try turning ur phone and pc off when studing so as to concentrate. u must always study in a quite place. i was just like you,so am telling from experience.