Speak as educated nature suggests to you, and you will do well, but let it be educated and not raw, rude, uncultivated nature. Demosthenes took unbounded pains with his voice, and Cicero, who was naturally weak, made a long journey into Greece to correct his manner of speaking. With far nobler themes, let us not be less ambitious to excel.
Those who mistrust their own abilities are being too wicked to themselves, discouraging themselves from doing what they should have been excelling in. If you are good at discouraging yourself, you can't be a good leader because leadership is built on inspiring others to face challenges.