Aang was a naturally gifted airbender and a master at a young age.
Once his avatar nature became known, he actively ran away from it. Then, when thawed. He learned to embrace it.
Also remember the journey Aang took. He only really had trouble with one element. Earth.
Air came naturally. He didn't need to think and barely needed to train it.
Water also came easily. To the point that he was doing things Katara took all her life to master that she said was hard, and he did it like nothing.
Fire, too, came easily. So easily that he treated it like a toy and burned Katara. He then placed up a mental block and refused to use it again.
So two elements he could use easily, one he could use with ease but chose not to out of fear, and one he struggled with.
Korra was the same. She was fine with Water, Earth, and Fire. But she struggled with Air and even outright dismissed it at first because "I'm already good at these three. Why do I need the fourth?"
She was exactly the same as Aang was, only more willing to embrace the three elements she was good at.
Korra had a lot of problems. But tony Korra being a little spitfire wasn't one of them, nor was it any more mary sue than literally any other Avatar.