What skill(s) am I missing that hinder my giving more meaninful responses verbally?

Basically I feel stupid. Sometimes I just find myself unable to process a conversation and respond with anything but empty words. It’s like I’m talking without truly saying anything. To my frustration, I recognize the matter immediately, but upon realizing the fact, my mind seems to withdraw still further. On that, I sometimes introvert entirely and go off to think a steady calming nothingness.

Some people I know spout off meaningful articulate dialog without falter. It often leaves me feeling like they’re fully present and thinking quickly, while I’m off elsewhere thinking slow as a rock.. In those moments, my responses merely serve as a voice to perpetuate, instead of contributing to, or driving the conversation.

Maybe the truth is that I’m just plain not quick enough (aka a big ‘ol dummy), in which case there’s not much I can do about it. Since that’s a rather unproductive approach, let’s just pretend I’m smart enough… What/how should I practice overcoming such a deficiency?