Not too long ago I conducted a seminar for 300 people. The person who organized it asked me to do the seminar for the entire second day of the three day event. It is true that I do that regularly, but this guy was so worried about the three day event going well, that he really tried (probably not intentionally) to put me under some serious pressure. He told me that there were big expectations for my day, that the speakers and seminar leaders in the past were all very good, that some of the people in the group were very tough critics. He even called me the day before and asked me if I was ready and that the guy before me was doing a great job.
I thought to myself that if I were insecure, I would be worried, but I wasn’t! I thought to myself that this is MY seminar and I know this better than anyone else. No one knows this material as well as I do. I also had done it before and knew what to expect. I TOLD MYSELF THAT I WOULD BE FANTASTIC! And guess what? They loved it. Fortunately I had the whole thing taped and I’m having a promo video made up.
So how does this relate to your situation? Maybe you haven’t done this before, but just like I was, you have to BE PREPARED! Practice over and over and over again and know your lines, know your character, know everything about her, know the lines inside out, backwards and forwards and upside down, know why she is saying them. BE your character and remember when you’re on that stage that it’s not about you!!! It’s about your scene partner. Put your attention on him/her. Really listen to what they say and then respond with your lines in a natural way. Sounds easy, but tell yourself positive things before you go on stage.
Be ready and you will feel better.
For a more, somewhat technical explanation, have a look at this page on my site for new and aspiring actors. It talks about how to overcome stage fright.
Good luck! You will be fantastic!!!