Speaking is a different skill than writing.
- Speaking is direct, writing is indirect (the paper/computer acts as a buffer)
- Speaking is in real-time and linear, where you have to allow mistakes to ride and correct them by weaving them into your later comments, while writing allows for editing and thus a "more cohesive" first product, you can edit spatially in a sense and mistakes can actually be eradicated before the reader sees them.
- Speaking involves lots of physicality -- the sound and tone and inflection of voice, body language, facial expression -- while writing is based far more on your ideas and what you want to present, you can much better control your "presentation" of self.
I guess my point is that they are different skills, so being good at one doesn't mean you will be good at the other right away despite both being communication modes. So don't feel bad; I am far better at writing, but after lots of practice in conversation with others, my speaking skills have improved tremendously, so it's possible to get past the fear and feelings of ineptitude.