This is very interesting. I've experienced a similar ... I don't know that I'd call it inability to focus, so much as an unwillingness. I can focus fine on writing, for example, but not on the paid work. I think. too, that when you attain a certain level of competence, you don't get the same kick from challenge and success that you do when you're less skilled. That in turn switches off the perky fear and joy hormones, so your ability to assess your performance becomes less sharp as well.

Of course, it's sensible to get checked out if you're worried, but I suspect it's all entirely normal and just your priorities shifting. I suspect the prospect of retirement on your near horizon is making work seem less and less important!