I feel like I'm stupid sometimes?

I know how you feel ,I feel like that too sometimes – READ – watch documentaries about things, what excites you?
Learn more about whatever it is that excites you – don’t judge yourself too harshly.
What you’re actually saying is – I want to learn and that in itself is an intelligent thing to want to do!! ;)

Start reading about things that interest you.
Take courses in things that interest you.

Never say “I can’t.” You can do anything if you just put your mind to it.

Don’t be like that. Tell yourself great things like I am smart if I apply myself. Don’t judge yourself to harshly about things. Pursue challenging mental hobbies like chess or memory games.

Pick one or two things you like and focus on them. Most people that we consider “intelligent” are really only knowledgeable about a very few topics. Many of our “intelligent” people only know how to present their opinions as being valid. I believe that the most “intelligent” people are the ones that are smart enough to know they don’t know everything, that listen at least as much as they speak.

What interests you? What would you like to know more about?

I agree with @seawulf575 suggestion to focus.
Snoop around and sense what subject(s) naturally attract your attention and point yourself into that. Read whatever you can about the subject, go to lectures, meet people who know the field and talk about it with them…

Read. Read everything. Get a subscription to the newspaper. And remember: the whole world might be a better place if everyone realized their stupidity sometimes.