I heard an expression once, one that seemed to sum up personal strength and confidence: owning oneself. This, to me, speaks of someone who knows themselves very well, and is honest to themselves about this knowledge; not that they are strong all the time, happy all the time, or live without any weakness or doubt. I think it all boils down to a certain sense of trust in the self: You trust who you are, how you'll feel, what you'll do, and are in little doubt as to how you deal with unknowns. You trust that you'll be able to handle whatever comes your way, and if you can't, you at least trust that you can find out how.