Take your lack of concentration as a sign that you need a new job or profession. I have to love what I am doing to stay focused. Having said that, I have not loved what I have been doing for a very long time. Hence I, like you, cannot stay focused. I try the work sprint approach--->try like hell for as long as I can, then coast. If I can sprint again, fine. However, I know I have to find something new. I can't do this forever.