I can no longer...

• do math
• retain knowledge
• read for more than a couple of pages
• form simple patterns
• solve simple problems
• play Touhou

Can any EXPERT PSYCHOLOGISTS tell me why this is?

>>1
Go see an actual psychologist. You're unlikely to find anything other than patients here, I'm just one of them.

A simple answer is that you are not getting enough rest, i.e. good sleep.

Depression commonly affects concentration and retention as well, or you may have ADHD. A school counselor may provide you some insight for any of both, if you are still a student of some sort.

I personally found out about my own ADHD into my twenties, last year, as I hadn't had problems in school before, probably because it was too easy until College. My poor concentration and performance in my previous years I just ascribed to laziness.

And don't discount other disorders; they go annoyingly hand-in-hand.