Attention Deficit Hyperactivity Disorder (ADHD) begins in childhood and often resolves in adulthood. Individuals with this disorder describe difficulty since elementary school with sitting still and focusing. They have a consistent history of easy distractibility; poor follow through on assignments, and forgetfulness. As a result, their grades usually suffer and they are often reprimanded by teachers for being too energetic or not paying attention. These symptoms often improve in adulthood, but may not completely go away.
However, in your case, your ability to focus and apply yourself has always been quite good until recently. While you may be having problems with attention, this is not consistent with the longstanding nature of ADHD. Impaired concentration can result from many different causes including anxiety, depression, medical illness or psychological factors such as being unhappy with your job. It would be best to get psychiatric consultation to explore the nature of your difficulty. The solution may require making some adjustments to your work habits or addressing an underlying mood or anxiety problem with medication or psychotherapy.