You don't have a speech impediment--you are just a fast talker! Slow down a bit and your mouth will keep up with your brain. Don't feel like conversation is a competition, take the time to enjoy listening to another person and then reply thoughtfully. You might find that if you slow down a bit, so will other people.

Of course, some of the people we know talk a mile a minute, and even if you're dead silent they will continue to talk quickly. Those people probably are not all that interested in listening to you anyway, they're just waiting for their chance to talk, so why bother contributing?