Roku told Aang that water was challenging for him to learn, which suggested that the opposite element of the Avatar's native element was their most difficult, but LoKgoes furtherbending by stating that the element that is the opposite of the Avatar's personality is their most difficult one. It is just that the Avatar traditionally lives the first 16 years of his or her life as a native of the nation he or she is born in. Aang pacifist thanks to hisAir Nomad upbringing, which is opposite Earth Kingdom's principles and ideals.Like everyone else said Korra's personality makes her stubborn and aggressive, whichare traits of Earth Kingdom and Fire Nation, which make it easier for her to learn earth bending and fire bending, despite fire beingthe opposite of her native element. That is why air bendingwas her most challengingelement.
Aang I do not think hadathat hard of atime learning fire to be honest. When Aang actually did practical fire bending he learned pretty fast. Sure he burned Katara, but that was an accident. He still successful reproduce the trick her saw that perform do. After that he was reluctant to learn it, because he burned Katara. He picked up what Zuko taught him fast. With earth he was faced with a element that was total opposite of his ideals that when the time came to do it he just couldn't until he learned there was no other way, but to meet it head on.
How Korra learned water bending is beyond me. Her personality is as opposite of a water bender as it is air bender. Water benders go with the flow and are willing to change. Korra is stubborn and a my way or thehigh way type of girl when we first her. I am not sure how that translated to fast water bender.