I had a lot of research chemicals (mainly Methoxetamine) ever since then my cognitive abilities have decreased. Seriously did some damage to me, not a psychotic break but I found my memory had suffered and the damage has lasted years.
I think that as we grow older and learn more about the world, we become a lot less confident in our own knowledge and ability. We feel dumber as we grow wiser. Depression can also be confused with cognitive impairment.
Since I've used recreational drugs, I've found it more difficult to entertain myself without them. It's easier to smoke a blunt and zone out than read a dense book or start a project. But it's important to remember how plastic the brain is. If you feel your cognition has suffered due to intellectual laziness, it's very much possible to regain your ability and then some. The biggest challenge is breaking out of complacency.