[awein999]

Can you elaborate some on what specifically is hard to concentrate on and what goes through your head? From what you say it sounds like you have attention deficit disorder. It's impossible to tell though based on the information you gave.
Other possibilities that aren't purely chemical could be lack of confidence, anxiety, aimless feeling.

Some suggestions are:
-Get exercise before you sit down to do work.
-Identify your purpose for doing the work: eg. do you want to? why? what is your goal?
-Work with your teachers after hours, it's a very helpful tool and it's underused by students. It'll force you to focus. They'll help you help them help you.