There may be no more unsophisticated, longer-lasting diagnosis than ADHD which was described two-hundred-years ago as "mental restlessness, later as Minimal Brain Dysfunction (MBD), and most recently as Attention Deficit Hyperactivity Disorder (ADHD). This, despite its diagnostic symptoms of anxiety, depression, and concentration difficulty being present in nearly all serious medical and psychological disorders.
Unsurprisingly, adult ADHD has now become a popular diagnosis, often resulting in treatment with a stimulant medication having potential side effects of seizure, stroke, abnormal heartbeat, mood changes, and more.
But concentration difficulty that has existed since childhood may also reflect a paralyzing fear of fear, a heightened sensitivity to anxiety against which normal ego defense mechanisms had never developed due to the early faulty interaction between parent and child. During thoughtful work, an individual is experiencing their sense of self, an ego capacity that can be deficient due to childhood deficits.
Relieving this deficit's crippling effect in an adult, in the absence of lengthy individual psychotherapy to replace their deficient ego capacities with more mature ones, may possibly be gained with the same technique which is used for battling many fears: by providing psychodynamic understanding, and engaging in the feared activity under a controlled circumstance.
Thus, one who has difficulty concentrating should practice concentrated work for a short time each day. As the fear of impending anxiety is reduced, their ability to concentrate may improve. Being cost-free and lacking dangerous side-effects, this technique is certainly worth a try.