I had the same problem, until one day I realized that it matters very little what you say.
Nerds, intellectuals have this notion that all social interactions must be perfectly structured, insightful, witty, impactful. It might be true if you are at a physics conference, or a Mensa meeting, but not with the rest, 90% of the population. For most, being social is all about being relaxed, being silly, and having fun.
So next time, just say whatever the first thought that comes to your mind. The fact that you are saying something, anything at any given moment, means that you are the center of attention, which in turn gives the aura of being relevant and social.
Bottom line, get out of your own head and stop worrying about how you look or sound like to other people, because it doesn't matter and people don't care. You'll actually be at your best when you stop being self-conscious. And over time, you'll get the hang of it.