When in a conversation, I know what I'm going to say, but I can't say it?

For instance, if I'm talking to someone I consider a superior, I know what I will say to them in my head, but when I go to confront them about it, my words come out jumbled and confusing. This also happens in normal conversations with things that I'm pionate about or if we're just talking about something casual like homework. It's also a big problem if I'm in an argument. It seems as though it takes me a couple seconds just to understand what they're saying, then respond with something much less than I hoped would come out of my mouth. Could I have a comprehension problem, or could it be that I'm shy? This has been something that I've had problems with for years, and I'm just trying to find a way to overcome it. :)