You must be careful not to let this become a self-defeating attitude.

Stop comparing with others and only focus on improving on your prior self. As long as you are making progress it does not matter how small each quantum is or how slow.

Concepts like "smart"/"stupid" etc. are relative (assuming there are no obvious biological causes) and are applicable only in specific domains. Each person can be associated with a "Radar Chart" (https://en.wikipedia.org/wiki/Radar_chart) of domain expertises (one spoke per domain) where it can be seen that one is naturally "smarter"/"stupider" in some than others.

Drops of water falling, if they fall continuously, can bore through iron and stone - Chinese Proverb.