Good thread. For me, it has been overcoming self-doubt and self-protective instincts. Accordingly, I have had to (and continue to improve on) bridge the gap between my analysis and execution. You can have elite-level analysis but if you don't trust it and execute on it, you're going to have disappointing results or underachieve.
Having a solid game-plan is not good enough in this business because it has to be executed in real-time markets. That in itself is a skill that has to be learnt, often the long and hard way. One of the reasons it's hard to properly execute is because of the fog of trading which is similar to the fog of war. https://en.wikipedia.org/wiki/Fog_of_war Same with sports if you think about it. The game-plan might be excellent but if the player(s) can't execute properly, it's not worth much.
The biggest challenge that I find in trading is knowing how much is too much and not to overtrade. Whether you are trying to capitalize more on your profitable trend or trying to reverse a recent loss, it is so tantalizing to trade more in bigger order size in order to achieve your goal faster without realizing it's overtrading and is posing undue risk for your trading capital until it's too late.
Managing your expectations. Avoid trying to be the best. Avoid trying to be right. Avoid focusing on making money.