I always try to exchange clothes with someone in the room. That'll change your mentality big time!
To me, the most important thing is understanding the copy and what its trying to convey. I'll bet it's not as bad as you think, even though your mind is telling you one thing. Plus, part of that radio confidence is what allows us to address the mic and make the words our own. A comedy mentor of mine once told me that the most important thing was reading a script and not make it sound like you're reading a script. Pretend like you're playing a character.