Well for me, I never think about "messing up". Before the presentation, I always imagine how successful it would be. I usually don't think about it, I don't think about who's watching, who they are, how big or little the crowd is, how I'm going to present, or anything. the more you think, it more stage fright you will get. Just walk on stage and do what you practice or end up naturally deciding to do. Just live the moments on stage and don't think about other's opinion of you. Most likely, your audience is encouraging you to do well because they wouldn't want to sit there wasting their time to listening to someone who don't even know what their saying