easy. Build something.
Yeah, that's was my first thought. But perhaps it's not quite as simple as that.
I've been a developer for almost 4 decades now. My confidence in myself ebbs and flows. But the pattern is highly predictable: when I develop something that works, it flows, and when an idea doesn't work out it ebbs.
When I start any project I always think it's a cracker of an approach, of course. Sometimes that's true, something it's not.
What that means is "Build something" doesn't always work the first time. But if you keep building things you will accumulate hits, and eventually confidence that brings.