If you rewatch Last Airbender, all the previous Avatar have to went through training to learn how to bend other element. Roku took 12 year to learn and fully master all the element. Aang, with intense pressure, master it within a year. Korra, without being taught by anyone how to channel the element, know how to do it by herself. Korra's situation isn't normal lol.

And what element is hard to learn for the Avatar is supposed to be dictated by the opposite element, not personality. Roku struggle with Water because his element is Fire, Aang struggle with Earth because his element is Wind. Aang didn't actually struggle with fire, not at all, he learn how to do it immediately after being taugh how to channel it, but doing it so carelessly and without discipline, even after being warned by Jeong Jeong, his Firebending Master at the time. He accidentally burned Katara and since then he did not use firebending, until he learn from Zuko. Legend of Korra changed that.