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Self-confidence/assurance is a feeling of trust or confidence in one's abilities, qualities, skills, intellect, brain, strenght, character and stuffs like that. Now listen up, you should be self-confident/assured. But, never be overconfident/assured i.e. never be excessively confident or assured. For, your ability/quality/skill/intellect/brain/strenght or a stuff like that can fail or disappoint you even when you least expect it or rather when you need it most.