[PeterisP]

If you state that you feel dumb(er), can this be measured quantitatively in any way? Can you re-take some tests where you know previous results to see if there are significant differences?

The described experience makes it hard to distinguish between reduced intelligence as such, versus disbalances in motivational/emotional systems that affect what you[r brain] choose to do, instead of the ability of what you can do. Both can be limiting, but in very different ways.