You need that self-discipline to make sure that you will practice whatever you setup before playing, else, you will messed around.
Once excitement is already there, to me, it gives me the thrill and wanting to gamble more. I'm losing my self discipline with that but it knocks me out whenever I start to have a losing streak. It's good on my end that it wakes up whenever I feel that way. Because setting up yourself before you gamble is needed that you'll have your own book and rule to follow. Easy to say sometimes that we can control ourselves but, once you're already on that momentum, I doubt that you'll have that control anymore.
But that's what self discipline is. You need it only when it's harder to keep your cool and make constructive decisions. It's not being used when things are going ok.
I would go as far that people who don't apply their self discipline when things are going badly, they never really had self discipline to start with, but that's not a diss.
I am guessing that level of struggle with it depends of individual, and some people are more burdened by their strong emotions that they aren't equipped to deal with.
And weirdly for some people that shows as weakness, even if those people are struggling more intensively and using more energy for that self discipline.