Well you kinda answered your own question. Korra finds Airbending difficult cause she acts like a stereotypical Firebender. Her personality is the antithesis of an Airbender, hot-headed, impatient and aggressive.
While I’m sure the normative trend was for Avatar’s to struggle with their classic ‘opposite’ element, because Korra’s personality is so overtly aggressive she was always going to struggle with being an Airbender. And did. Until Amon did whatever the hell he did. Or was it Mako? Who knows.