Most of the time, greed really push you to change the direction of your plans, there are position which you already reached your target but instead of stopping you choose to keep trying to earn more, and in time that the direction of the market turns against you, that's the time that you really regret why you choose to stay instead of quitting and withdraw the money that you supposed to earn.
Exactly, greed also can make you not to accept the initial profit that you must have already earned. Greed can make the trader to overstay in a position that they are supposed to have exited and take profit first profit but because they are expecting a huge profit, they will not exit the trade until they lose the first profit. Greed can make the trade to use a big lot size or a leverage size.
Yes you said something right about the impact when we are unable to ignore the greed that arises in the mind, simply the point is greed will make someone lose something they should have gotten before, so never try to exceed everything when it has become your portion.
I understand that we never know about the right portion size, but by having a plan and being realistic I think you should have a plan about when is the best time for you to stop and secure profits.
Actually on the other hand it is okay to apply greed as long as we are able to accept the fact of regret, but in most cases the majority of them always find it difficult to really be open-minded, so it is better to be grateful for any amount of profit as a form of appreciation for the efforts we have made in analyzing the market.