Have you ever considered taking your lack of concentration problem to a doctor? It may be a medical condition if you love the subject and you just can't concentrate. But, if you're bored with the subjects in all of your classes, you may want to do a bit of career exploring and change majors, possibly. I'm assuming you're in college.
At the same time, you may just have to suck it up and just do it. There are great articles on this site on
self-discipline.
I personally realized that I shouldn't be an accounting major because I can't stand studying the subject, and my grade in that class reflects that. However, I love the rest of my classes, so as soon as I finish the semester, I never will take an accounting class again. It was a learning experience that I really appreciate.