The first step is to avoid any thing that would make you emotional as a gambler. One thing most gamblers do that makes them end up being controlled by emotions us that they risk just too much. Remember that you should only risk that which you can afford to loss. And if you cannot afford to loss such funds, then risk very little portion so that the loss doesn't make you sober and end up emotional. We try our best to hold ourselves as gamblers because the moment you get emotional, you will end up losing too much.
Being a wise gambler can make you able to control your emotions. Most people say that emotions are difficult to control because it is not easy to do it. I agree but most of the causes are because you bet beyond what you can afford and you have the wrong motivation in gambling. For example when you have an income of $ 200 and you allocate a budget for fun around $ 20, I think you will not be greedy and emotional when you lose. But if you try to take risks, such as throwing away your $ 100 then you will not be able to control your emotions when you lose your money. And the second is your motivation. If your motivation for gambling is as an additional income, I am sure 99% of people like that will not be able to control their emotions.