For many people, feeling that they are stupid is not common. For others, they feel like they're constantly feeling plain dumb. This means they need to learn how to stop feeling stupid, possibly learn how to forgive themselves, and possibly learn how to think before they open their mouth.
Feeling stupid is often the result of something that was said that didn't make much sense. It's often because the speaker wasn't listening very well to what was being said, or they opened their mouth before they realized what the conversation was really about. This results in a feeling of being stupid, when it's not really stupidness or even ignorance, just reacting before thinking.
If you're one who often winds up feeling less intelligent than you should be, stop and think before you open your mouth. Often, the conversation is deeper than you recognize it, and that's why when you to jump into the conversation, and wind up feeling stupid. In other words, think before you speak.
If this happens to you a lot, make a promise to yourself not to open your mouth without thinking. As you get better about not speaking before thinking, your feelings of stupidity will lessen.
The other side of the conversation is those people who feel stupid no matter how smart they are. First of all, no one in the world can know everything, most people have a general intelligence, but many people have a specialized intelligence in certain areas. If you find that you're feeling stupid, find your special area of intelligence and learn everything you can about it, and don't worry too much about not knowing much about the rest of it.
Feeling stupid is not a natural emotion, if you find that you're feeling stupid on a regular basis, try to learn how to think before you open your mouth.