Yes, discipline is easy to say but not easy to practice, even if you realize it. I've experienced this when I was betting on poker and I realized about discipline, initially playing with complete control to occasionally take a break when betting even though it was a win. But in the end I felt if I continued that I might get bigger wins, so I lost in a very short time. So yes, discipline is only easy as a theory, because in reality if we still can't control our passions then we will still be trapped in defeat and that is also a sign that you are addicted because you will be obsessed with taking back what has been lost.
Most gamblers have that attitude, thinking that pushing your luck and try to win more instead of allowing yourself to bag
some profits and enjoy after quitting.
Nature inside that greed really plays such a role, giving you that mindset to try for more and forget about your limitations.
Discipline, emotion control, bankroll managements, everything is good when you are compiling your strategy, but once you are into executions, things always change, either too much emotions or heavy greediness that fills up inside you.