Shyone, it's easy for us to see that you are not dumb, and it's also easy to see that you are being run by this habitual thought, "I am dumb." I coach people with limiting beliefs like this all the time, and I can assure you you're not alone with this; it's a very common one.
It's easy for us to see "I am dumb" is not true, and my guess is that when I say that, on some level, you think I'm dumb for saying so. The thing is, when you hold a limiting belief for yourself, you are holding it for the world. Same is true for an inspiring belief -- it's not just for you, it's for everyone. Can you see where by believing this thought you're not just limiting yourself, but also limiting possibility in the world?
BUT -- I'm not going to try to convince you that you're not dumb (It wouldn't work. It never does.) I would like to invite you to consider, though, that this belief comes from a particular moment -- something happened, probably when you were very small, someone said or did something and you made a decision about yourself: "I am dumb." It's a thought in a little kid's (your) head, and it has been running you for years. We spend tons of time simultaneously trying to prove that our belief is not true, and trying to avoid having people find out it IS true. Eventually, you get exhausted! And some bold people break down and say: here's what's so, please help. We've all got these limiting beliefs, but not everyone takes them on so boldly -- and you are very young to be doing this, so I would actually say you are on the fast, smart track!
Can you remember the earliest time you felt this feeling of slowness, failure, embarassment, shame? What happened? What time of day was it? How old were you? What was said?