I note that you are 18 years of age. This would be best addressed by a thorough assessment by psychiatrist
who will enquire as to how long you had this problem, whether your studies been affected throughout school years because of this issue and if you have any other problems or fears or worries along with this. A whole lot of similar questions need to be asked to understand your problem before appropriate advise can be provided. Please consult a psychiatrist or psychologist
as it is important to improve your focus and concentration on your work. In the meantime you may try meditation
, sudoku or such strategies to improve your focus.