> if I admit that, not only am I not confident, but I see the world as so complex that I can't be possibly confident about it, how can I decide what to do?

You get input from others with knowledge, consider things as much as you can, and recommend a course of action / make a decision based on what you know, understanding that it might not end up being the right one.

I've seen it happen often in small businesses and startups I've worked with, and I've seen people make what in hindsight were the wrong decision. But that's just the way things are for everyone, really.

You can't see the future but have to make a decision anyway at some point.