One of the areas that I have a lot of discipline is money. I make an annual and monthly budget, and part of the challenge is to come under budget, which I almost always do. Partly because it's a very generous budget and partly because I'm not a good shopper/spender.
Where I struggle with discipline is eating. Over the years, I've learned what I can get away with food as long as I exercise. "What I can get away with" isn't discipline.
For the past two weeks, I've been keeping track of everything I eat and trying to stay within a reasonable calorie budget (not super low, but low enough to lose a reasonable 2 pounds per month). The first week was really hard. I was going above my budgeted calories, and at the end of the day I'd try to fit in a lot of exercise to burn enough to get down to my calorie goal. The second week has been a little bit easier. I'm starting to feel when my stomach has hit the calorie goal and then trying to stop for the day. I'm not feeling as satiated as I am used to. So there's a lingering emptiness at the end of the day, which is both a psychological and physical obstacle.
The funny thing is that I'm not doing this to actually lose weight, which of course IS a great side benefit. The real reason I'm doing this is trying to create more discipline in myself and being mindful of the decisions I make and the repercussions of them.
We'll see how things go.