Yes, the point is self-control, that's what will make it easier for a gambler to make rational decisions in any situation, such as after successfully achieving victory.
Actually, greed is a choice, in the sense that you are free to apply greed but with the note that you must be able to accept whatever the results are, especially losing the amount of victory that you have previously obtained.
But yes, I am sure that it is not easy to accept a losing situation when you have previously succeeded in achieving victory, and this is why stopping and securing the money is always the recommended decision because that is the best way to minimize all unwanted possibilities.
Yes, having self control in gambling is a must so we can be wise when making a decision especially when we wins big money. We will not trying to chase the other winning but we will stop gambling and withdraw the money. We know that we don't have a big guarantee to win in the next rounds so we know what we need to do. We can also prevents the greed becomes bigger and stopping ourselves from gambling will be the best decision.
It is indeed not easy to accept the lose situation in gambling and that can attract us to use more money and try to wins. But when we know that will not be easy, we will not trying to keep playing gambling.