If thoughts of self-control are constantly flying out of your head during the game, you can make a big note to yourself. For example, draw something large and bright on a piece of paper, it can be the word “Stop” or some symbol that will immediately remind you of control. And while you are playing, this note should always be in your focus. Accordingly, if your brain learns to react to the note in time, then there will not be any serious problems with the game and you will not have to over-emote. That's the idea.
What makes you think that the person wouldn't get rid of that note once they lose control? We know how people with mixed emotions behave; they get frustrated, angry, emotional, and they rage and out of that rage, they can do anything in the heat of the moment, so I don't think a sign saying "stop" can actually stop them from gambling because a single word written on a piece of paper wouldn't calm their nerves and it won't serve any purpose at that moment, it might even increase the anger of the person if they have lost a lot of money already.
In my opinion, self-control is not something that can be learned but a natural thing that comes with your nature as you grow up. It might also depend on your genes and we often tend to have some genetic qualities and properties inherited from our parents and elders, so if it's in your genes, you might have a calm and controlled nature, but if it's not, then it is going to be difficult for you to have a grip on it.