We only know what we already know. We don't know what we do not know. How do we know what we already know? How do we know what we don't know? How do we know what we never know?! "What is the Most Important Thing in Trading?" I guess: The single most important thing in trading would be of course 'How do we know "What is the Most Important Thing in Trading?"'. Ha!
Knowing that there is no "holy grail" or only one special "something" to help you trade. The most important thing about trading is to develope a well rounded entry/execution/exit with proper risk management and KNOWING WHERE YOUR OWN PERSONAL WEAKNESSES ARE. If you don't know where your own personal weaknesses are in trading you will keep making the same mistakes and never have success, no matter how great your trading style, technique, money management or whatever.
Something else: luck, fate, destine, how much you can earn and when it happen is more or less fixed, regardless of trading or other things you do