being "confident" doesn't mean you should double check just to be sure, it means you should pretend to be certain about things you are not certain about.
I've spent some time reflecting yesterday and I realized that. When people want you to be confident they don't want you to be actually confident, they want you to pretend you are. It is idiotic and makes no sense, but it explains a lot of situations in which I behaved the wrong way.
confidence to me means the opposite of that. it means questioning your asssumptions, approaching things from a different angle, reflect, recalculate, asking for a second opinion. Because I'll end up with greater confidence that my assertions are more truthful. But apparently doing all that makes people think I'm insecure. Shit!
Double-checking everything you do is the opposite of confidence. You may gain confidence from the checks, but you're not confident in your initial choices.
It's important to understand that nobody expects you to be fully confident 100% of the time - nobody who matters, anyway. So don't feel too bad double-checking stuff, and if anyone gives you any guff about it just start crying and shit your pants and they'll go away 👍