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The best gift you can give a child is to genuinely believe that they can become and achieve so much more that they currently can.
They will feel it and they will be inspired to try harder to match the expectation and they will find their own self-confidence along the way. That is impossible to do unless you are already a confident adult too.
That is why insecure adults will answer to them instead. You can't believe in others unless you believe in yourself first. And self-belief comes from discipline, effort, and anxiety. No self-talk involved.