How can I become more comfortable speaking in cl?

I am an English major and much of the curriculum revolves around discussions -- most in which I don't participate . I find it overwhelmingly difficult to articulate when I am called upon. It's even hard to speak when I am certain I have something profound to say. There are times when I have been called upon to speak and have made clear and thoughtful statements so I know it is in me to be able to speak. How can I make it easier for myself to speak in cl without being afraid and losing my thoughts?