It's hard to separate out the deterioration in critical thinking and ability to focus on a task into it's various causal factors. It is probably multifactorial and comes from
a. various psychiatric medications taken at high doses
b. a number of manic episodes where each one leads to further deterioration in the brain
c. increasing and enduring social isolation
d. aging
e. brain fog from chemotherapy
f. long lasting and deep depression and fatigue

I used to be a professional who used my brain to make a living. Now I can barely type these words.