Perhaps I missed it above but do you take medication? It is not uncommon if so that one will feel 'dumbed down'.

I myself notice tasks I was once very apt to do well - problem solving for example - a great deal difficult.

Doing math operations and completing puzzles (sudoku and crosswords) seems to have improved this. I even download elementary level arithmatic drills in order to keep my mind sharper and quick. Still, the medication has taken its toll - but these actions seems to help.