Confident?
Who told you I was CONFIDENT??
I think it's important to remember that there will always be room for improvement. You're always going to have a better idea, you're always going to look back and have some sort of regret on what you could've done to make your story "better".
Problem is... if you're aiming to create the perfect, most flawless beginning... you're never going to start, and if you never start, you'll never improve! If anything, you'll be constantly stressed about NOT starting. Like I was.
I'm a firm believer that the "just start" ideology for comic making is flawed... does a scuba diver just jump into the ocean without their gear? NO!!! Comics require the same kind of preparation. However, if you find that you're dwelling on creating the Best Idea Ever, it's probably time to stop procrastinating (and yes, going back to perfect what's barely been started is a form of procrastination, believe me) and push yourself to begin.
With my comic, I've had the story brewing in my head for at least 2+ years, and in that time I've been designing characters, writing scripts, rewriting scripts, redesign characters, drawing full-color pages, scrapping them... I've been through so many beginnings that there are certain characters and scenarios permanently seared into my memory. I knew what I wanted, I just wasn't sure how to get what I wanted. Like you, the beginning was the hardest part; I wasn't sure how to introduce the world without taking away from the story, and I constantly found something wrong with the composition of the pages and/or the story itself. I finally broke down and posted a cover page this month (as well as scheduled my first update) and you know what? It actually broke the cycle. And I feel pretty good about it.
Often we are our worst critics, and sometimes you have take a chance, despite your fears or desire to "put your best foot forward" and just write, draw, and post. And repeat. And repeat. You'll find your work was good enough as it was.
The important thing is to know your story, and have a basic idea of how you want to tell it as a whole. DON'T GET CAUGHT UP IN THE DETAILS BEFORE YOU EVEN BEGIN
my advice would be to take the best parts from all your scripts (hopefully you saved all of them) and compile them into a final script, and start. or better yet, just start with what you already have. save that eagerness to improve for future pages, because that's where you'll improve the most anyway.