How do I become less anxious in conversations?

I find myself being very outgoing most of the time, but there are some times when I don't know what to say. I don't even want to talk at all. It's like, I search for the right thing to say and nothing ever comes out. I don't know if I'm outgoing only to fill the void of not knowing what to say. I don't know if people like that about me. I'm very self-conscious and always think people are talking about me, so it makes me have a cold shoulder and not want to talk.