All I can say is be sure that you actually do have ADHD before medicating.
Why you feel you can't concentrate on some things, but on others, can also be explained id you are an obsessive compulsive personality type with anxiety disorders. Explore "anxiety avoidance" ( https://www.meridian-counseling.com/blog/what-is-avoidance-b... ). At its extreme, people trapped in the vicious cycle of anxiety avoidance often even convince themselves that they have some other serious kind of issue (as a maladaptive coping mechanism to justify to themselves why they are giving in to avoidance behaviour).
Why you feel you can't concentrate on some things, but on others, can also be explained id you are an obsessive compulsive personality type with anxiety disorders. Explore "anxiety avoidance" ( https://www.meridian-counseling.com/blog/what-is-avoidance-b... ). At its extreme, people trapped in the vicious cycle of anxiety avoidance often even convince themselves that they have some other serious kind of issue (as a maladaptive coping mechanism to justify to themselves why they are giving in to avoidance behaviour).