It is a very subtle difference between the two words. They sound almost similar and start with the same syllable.
One word describes your personality and the other describes how you operate. Confidence and Competence are the key attributes of our personality.
Confidence comes with experience and Competence comes with practice.
To be confident, one must adhere to the rules of the game and understand how to execute learnings, and to be competent, one must practice the execution.
Confidence is being self-aware; being aware of what you know and what you do not. Acceptance of the unknown and willingness to explore the uncharted path.
Competence is accepting the ability of coming together of forces which en-kindle the thought.
Confidence AND Competence will make you Impregnable, which is what will make you cogent.