I believe my confidence came from two things when I started:
+ I wanted to see the story exist in the world
+ and the only way this would happen is if I made it.
You lose confidence halfway if you try to judge and edit your work while still in the midst of creating the story.
The mind space for creating a story is expansive, imaginative, and just loosely controlled enough that you stay in range of your story but still allow yourself to explore.
The mind space for editing a story is cutting, tightening, making things concise, and weighing and ordering each scene so that it serves the story overall.
These two things move in opposite directions so doing both while writing is like going forward, stopping then going reverse and stopping and so on. You may still stutter-step ahead but it'll be a jerky and disconcerting process.
Nowadays I gained confidence through studying story and how it works for a visual format. Comic writing is similar to screenwriting so I recently finished Story by Robert McKee and man did it open my eyes to a whole new way of thinking about stories.
Confidence is a mindset. Don't you feel confident when you have a sense of what you're doing? Rather than feeling out of your depth? My advice is to earnestly study the foundation of Storycraft, especially screenplay and stories made to be visual.
I have a small series of threads on which I'm summarizing my notes on aforementioned book. If you'd like to check them out to see if you want to learn more from the book here are the links: