I've never been in a play, but I have had to speak in front of an audience a number of times and can tell you what has helped me. I tell myself that I will just have fun with it and not worry about the outcome. When I take away the seriousness of it, it makes it easier for me and even mistakes don't trigger as much. It is the worry of the "importance" of the outcome that throws me off. Focus on making it interesting to the audience even if you don't get a line right.

I hope it all turns out good for you.