Are you a guy or a girl?
I think that for a girl this would be an advantage, actually. Men always have trouble understanding women, because they're less direct about their feelings than men are, right?

Also, being less open doesn't have to mean lying about who you are. You could also just be a bit more reserved about the things you tell people.
I'm not sure if it's a good idea to try to become less outgoing, though, because I'm on the "shy and introverted" end of the spectrum, and it's not so much fun here either.