Well, I got my degree in theatre so I might be able to help (yep, I'm just that old)...
Caitlin,
One acting class is not going to turn an introvert into an extrovert overnight. You've been shy for how many years now? Shy is not a problem, but it seems to me like you're putting far too much pressure on yourself. Are you a person who can go to a group of people, shake someone's hand (or whatever kids do these days) and say "Hi, I'm Caitlin" without prompting? If not, let's learn to take the small steps before we sign up for the marathon. To be honest, you first need to be comfortable with yourself. You are who you are -- your life thus far is the sum of your experiences. Have you taken the time to sit down and figure out who you really are?
Now as to why are you screwing up your lines -- to me, it's because you're nervous. It's more than being nervous about the lines themselves because I bet you know them cold. Instead, you're probably nervous because you feel that you're being judged by others. Guess what -- you probably are being judged but I'm going to let you in on a little secret: It doesn't matter! Really. (I know, easy for me to say). Who cares? Or better yet, why should you care what some classmate thinks?
Instead of worrying about your next line, are you actually listening for what the other actors are saying on stage? In theatre terms, it's called "being in the moment." You can't act, you can't speak unless you're "in the moment." Odds are very good your character probably hadn't sat down that morning to write out everything they were going to say that day. Instead they listened to what others were saying/doing and responded. Are you responding or is your brain already three pages ahead in the dialog? They say great talkers are great listeners. The same goes for actors. You can't trade lines with someone unless you listen to what they say and respond. Don't focus on your audience -- that'll come much later. Instead focus on the person your character is dealing with at that moment.
