I smoked tobacco cigarettes for 45 years. Every time I quit, I would get sick. What a coincidence! When I started doing some research, I learned that nicotine is very effective at warding off depression and anxiety, and also improves attention, concentration, visual memory, and hence performance. No wonder I felt so blue, so confused, like I was literally walking around in a fog when I abstained from nicotine.
I managed to stay totally off nicotine for 6 months, and kept waiting for the recovery that never came. Oh there were a few improvements in some things, but not in my thinking...