What I meant is that if you hold a royal flush then you are risking nothing on the hand. You will win (or at worst, I guess, share the pot). That's as confident as it's possible to be, objectively. Consequently, I think most people would also feel pretty confident playing with a royal flush.

Thinking about it more, you can of course blow a royal flush - you can appear too confident and scare everyone off. So there are still things to be insecure about in that situation. But there's absolutely nothing to lose by playing.

I wasn't considering how I'd got the royal flush in the first place. If I were to think to myself, "gosh, I know I have this royal flush, but can such amazing luck hold to the end of the hand?", then I'd describe that as an irrational effect undermining my confidence. Such irrational fears seem absurd when applied to a mathematical certainty like poker, but are quite common in real life. I think that confidence-as-a-personality-trait is the sum of such things, since generally it manifests as failing to feel confident in a situation where in fact the risks are non-existent or manageable.