|Image (borrowed + repurposed) via Bruceelkin.com.|
True confidence comes from within. It cannot be bought, it must be earned. As Dr. Brett points out, confidence and mastery are the byproducts of effort, failure, and learning along the way.
"Genuine confidence comes from hard work and experience. Putting in the time to achieve mastery and getting knocked down enough times to know, deep in your soul, that you have what it takes to succeed." - Brett Steenbarger
Self-knowledge may be another aspect of confidence. Is it tied together?
Have you achieved mastery in your field or in an area of life you are passionate about? What did you have to go through along the way? Are you still working towards your goals or expanding your "limits"?