When I was 17 I was exactly the same way. I remember applying at a book store when I was 17ish, had to have a friend ask for the application for me because I couldn't get any words out. Man did I feel like a loser.
You're missing an important fact here. Even though you were scared to death you still asked for the application. That takes alot of courage, believe me, I know. It doesn't matter if you choked, it only matters that got the application.
You're being too hard on yourself. Now fill out the application, go turn it in, get an interview and blow it. Then go grab another application, get another interview and blow that one as well. Eventually it won't sting so bad and slowly it'll get better
It doesn't matter if you blow it, or act insecure, it only matters that you have the courage to go through with the plans you make anyways. And obviously you do have courage because you got that first application without having to have a friend get it for you.