Emotion and Greediness are both harmful to traders. But, Both are part of human nature. You have to control yourself from emotions and Greediness. These two things can make you lose. Suppose you are at a loss; your feelings will force you to trade more in the hope of recovering the loss, even if you know that you might lose again. You have to control it and do research before you open another position.
Even if you are at a profit, greed will force you to open another position like before, in which you made some profits. You have to understand that the same strategy doesn't work all the time, and if you are a trader, you already know that.
So, I would say, You have to have control over your two human nature; Emotions and Greediness. Rest depends on skills.
I agree that it is going to cause a bit of trauma to people if we keep saying that they shouldn't really feel what they feel. If you feel greed, or fear, or happiness, or excitement or whatever, if people are telling you to "not feel them" do not understand it as try to not feel anything.
The logic is that you are going to feel these no matter what, we are all humans and we feel, the point is that you should be trading as you always do without putting these feelings into consideration. Just because you are greedy, doesn't mean be greedy when trading, have different points in time when you feel greedy and that would be a lot better for you in the end.