Ventus

Terra and Aqua had their Mark of Mastery exams today. But only Aqua passed. The Master said Terra couldn't control the Darkness inside him but I don't understand where the Darkness comes from. Why would it even be inside Terra and how come Aqua could do it and he couldn't? They're both great Keyblade wielders and they should both become Masters. Like we're all going to become Masters.

Any of you you guys ever battle with the Darkness inside you? And if you do, how do you control it? Maybe if I got some answers, I could help Terra for the next time.