I can't concentrate. Why not?

No data=no opinion. You get the most from this site when you provide meaningful background information on your issue & ask a related question. Unless you had ADD as a kid it is unlikely you can apply the label now. You preform best when well rested, well nourished, free from illness & in a comfortable setting. Bring a list of your issues to your doc & get evaluated. Over time the triggers should become evident.
Get evaluated. There are mant reasons for impaired concentration , includind adhd, depression, anxiety and other disorders. A psychiatric evaluation would help provide an answer.