Easier said then done though
It is difficult to have a limit when you want to get back your money after a loss. It's extremely tempting for most people to put more money in again without really thinking. I think if you lack discipline you shouldn't even gamble at all just like you say because it really can't be helped.
I do gamble and I can limit myself especially even my extra money is losing I will actually stop, because I don't really want to lose a lot of money that I cannot afford to lose. So I don't think you cannot control yourself.
That's true, some of us can't deny that there are also a few of us that can also control ourselves, yes, we all have greediness. But once I lose my money I don't want to lose again instead I will just save it.