How can I get over my stage fright?

I'm in an advanced class for Language Arts and Social Studies, and we have three separate plays. One is the Imaginary Invalid (it's French if you don't know what I'm talking about) a male Julius Caesar, and an Amazonian version of Julius Caesar. I tried to get a part in the Imaginary Invalid, mostly because I do better in comedies rather than dramas. Of course, my stage fright got the better of me, so I got put in the Amazonian Julius Caesar. Me and another person are the Soothsayer together. This is where the crappy part comes in. Not only do I suck with dramas, can't make some witch-type voice, but I'm too afraid to even say it out loud to the people I'm acting with! I think in my mind "I'm going to do this, I'm gonna make this play work!" but my body won't do it. I'm scared, and they all try to encourage me, and say I need to be louder, but I just can't!!!!!! My L.A. teacher said that she liked the difference in our voices, because the person I'm working with can muster the courage to actually say it loudly to them, but I just barely murmur it. I haven't a clue why I'm scared to say it, but I just am! Maybe if I asked if I could whisper it loudly, but I think that would sound kind of bad... What do you think? Help!!!!!!!!!!!!!!! :(