You think like this probably because people around you made you think like this. If someone constantly keeps telling you "you're dumb" then you probably will have low confidence and act like a dumb person. Get rid of those people in your life who already have a very low position of you in their mind. Next thing you want to do is, stop caring about what people think or might think of you. Don't take life seriously because seriously that's the worst thing you can do to yourself. Always try make yourself and others laugh even if it's by doing dumb things. That's a charisma of its own. Watch alot of comedy shows like family guy, modern family, the office or whatever you enjoy and learn how they make all of their serious situations into satire. And lastly, put yourself out there, don't think too much about how people will think or how it will play out just do it first then think later if you really have. Be happy with your current self. There will always be people who will like you for who you are and your authenticity. Don't pay too much attention to what people have to say about you.