Ok, it looks like we're doing this again. 1. Being able to bend an element is not the same as mastering it 2. Aang managed to figure out how to bend each element, including Earth, the one he struggled over most, in a single day. 3. Yes, Aang needed a teacher. But critically, he didn't try until he had a teacher instructing him, and once he did, he succeeded pretty much instantly. It's entirely possible he could've taught himself if he didn't feel like he needed to stick to tradition None of this contradicts the established canon of the Avatar universe, and it's entirely realistic that a young Avatar deciding to try to bend other elements on their own might succeed at doing so.