On a realistic note. It does scare the living crap out of me. I see what my grandmother is going through right now, and man with my siezure who knows where I'll be down the road.
Go get yourself tested for early onset Alzheimer’s. They do a cognition test as part of it and it may help you understand what is going on and likely put your mind at ease.
My guess is that your brain is just lacking excercise and you haven’t recognized that you need to work it harder to constantly tune it up.