@adrian888 depression, even mild can interfere with concentration. I had major concentration issues after each of three major depression episodes. I found a good psychiatrist and psychologist to help me out of the major depression. The right medication combination completely relieved it. Once the depression was under control I started working on concentration. Each time I found reading children's poetry, then preteen books and finally a novel helped me tremendously. If you aren't a reader, I imagine watching kids programs might be a place to start.
I know this has been a long post so I hope some of it helps.