Lack of concentration is a common side effect of depression. I quit a job once because I couldn't concentrate enough to get things done. I've also had similar problems at university. The job thing was so bad I began to think that I had ADD. But now I'm more or less convinced that it was depression. Looking back at my life, my lack of concentration, drive and focus has been episodic, and more based on feeling down and unmotivated, rather than a lack of ability. When I feel ok and reasonably motivated, I can usually concentrate just fine. Also, if you've been able to concentrate before, but getting noticeably worse now, it's unlikely to be ADD since that starts in childhood. Your lack of concentration should have been a major problem since you were a child.