Sup Aqua,
I'm glad you asked this for a rather selfish reason: It reminds me that I'm not alone in feeling this way from time to time.
So in regards to your question, here's the TL:DR version of the 4 reasons everyone should be confident in their work:
1. Confidence is fleeting.
2. This is the worst you will ever be.
3. One Finished Mediocre Novel will always be better than 100 great unwritten ones.
4. You don't have a choice.
1. Confidence is fleeting.
Motivation, Inspiration, and Happiness all share a certain similarity as confidence: They're all in an eternal struggle with their negative counterparts. There will be days where you will lounge around in apathy, there will be times when you will feel severely discouraged, not a single person in history can boast a sadness-free life, and even the most confident people have insecurities.
Even if you feel confident enough to post/publish/share your first chapter, I can almost guarantee you that sometime in the future you will look back on it and cringe. That's okay. It means you're improving, and contrary to what society would have you believe, it's perfectly fine to feel negative emotions; that's just part of being human.
However, due to it not being a constant, but a variable in one's life; rather than having confidence be the fuel that drives you to create, make the need to finish be what drives you to begin.
Organize all of your ideas from shortest to longest, and begin completing them one by one like a checklist. Do not touch any other idea until the one you are working on is finished. Eventually, the focus will be shifted permanently, and confidence won't even be a factor in creating and sharing your stories.
2. This is the worst you will ever be.
From the wise words of a not so wise Koala,
"You know what's great about hitting rock bottom? There's only one way left to go, and that's up!" -Buster
The only way you could get worse is if you stopped, and I doubt you are going to stop anytime soon. You should take pleasure in the fact that one day after a hundred stories, you will wake up and find yourself with an extended bibliography â with each instance of your improvement forever logged â along with a group of amazing individuals who fell in love with every entry.
Even if you're nowhere near where you want to be now, you will be. Just don't stop.
3. One Finished Mediocre Novel will always be better than 100 great unwritten ones.
One of my greatest fears is being known as the 'idea guy'. I'm sure you know what I mean. Being what essentially is a wordsmith, it genuinely terrifies me to the core that there is a possibility that people could find what a say empty.
I hope to never tell a friend what I have in store, just to be met with feigned interest. I do not want 10 years to pass, and be the sole expert on 100 different worlds and their rich history and culture. I do not want to be the person who claims to be a writer, but has nothing to show for it.
No matter how terrible it may seem, something is better than nothing.
Steven King is my favorite example; while he has written some iconic novels, he's also produced some stuff that I would probably not recommend to a friend; but his less than stellar titles haven't stopped him from producing over 50 novels.
4. You Don't Have a Choice.
I apologize for the way I have worded this final point, but I truly believe in it. You do not have a choice.
If you never start your project, then no one will ever see it.
If you never start your project, then you have, by default, failed.
If you never start your project, you will forever be left to wonder how different it would all be if you had.