well, its quite possible that your just not "meant" for being in plays. I do the same, but I have never felt nervous... Ever. I can't really understand what you guys (and girls) worry about. BTW i <3 books 2!
When you try out, or need to do anything which requires putting yourself in front of people, just imagine that you are playing the part of someone supremely confident who has been assigned this role. Putting yourself one step removed from the reality of auditioning and performing really does help. This little trick has made it possible for me to get through singing in front of nearly a thousand people, and countless public speaking engagements when I worked for MSU Extension.