Grrrrrr. Yes I never end up sounding as articulate as I do in my head, or even on paper.
I recently was talking to a friend about this who is similar in this regard.
I'm such a perfectionist with my writing, at least when it comes to word selection and I want my speaking to be reflective of that too.
So, I end up stumbling and stuttering trying to grab hold of that perfect word to complete my expression.