How to be sure if i can't concentrate and study well whether because of not having enough will or because of some biological reason to lack of focus ?

What KEEPS focus?: I bet there's some sort of book or some internet site that lets you keep your focus -- if there is, i'd not look for a biologic reason, especially if your physical ; routine labs are normal. At higher levels, staying focused on book-work requires active learning -- constantly answering questions ; doing problems; try this approach ; be wiliing to work hard. Your physician's your best guide.

I beileve that: You need to look the longitudinal course of your problem since childhood. Was there any problem in elementary , middle school, high school , college . Were you told that you are not focusing , turning in your homework in time ? What I am driving at is if you have attention problem . You need to talk to your family , friends , look in to your past and if there is a pattern , consider seeing your doc.