>>54471 (OP)Unless you have some malnutrition or metabolic disease, there is no simple cure.
Be more active both physically, intellectually and socially, do things that challenge your brain and give it new impulses. Eat healthy and lead a healthy lifestyle. Sleep enough. Just try to eliminate as many factors as you can that could negatively impact your consciousness and thinking and use your brain more so that it's less rusty. I'm not going to assume that's the case, but if you have some past(or present) troubles or trauma that weigh on you, take care of those. (Obviously)
Alternatively or better in conjunction you could use various substances to boost neurotransmitters in your brain, which can both directly help with brain fog, and help you be more mentally active and trigger neurotrophic factors which can improve brain fog in the long term.