There are so many factors that could be contributing to this feeling: age, life circumstances, drug use, diet, sleep habits, emotional overload/depression. One of the things that is very frustrating to me is that age (I'm 61) is definitely starting to diminish my sharpness. I can still get where I need to go mentally but it takes me longer to get there! What helps me the most is focus. I have a habit of thinking about at least 5 things at once--it's like being in a crowded party with 5 people trying to talk to me all at the same time. I need to focus on one thing and quiet the rest of my mind and then I feel much more capable and clear. Beyond specific factors like age, etc. diet is perhaps the most important. Make sure that your body, which includes your physical brain, has all the nutrients it needs to function optimally.