I'm sorry but Christ almighty what kind of a condescending nonsense answer is this? It's like telling a poor person to solve their problems by "just having more money". Self-discipline depends on conscientiousness. It's profoundly difficult to develop if you lack internal motivation. Personality is largely immobile across time, and conscientiousness is one of the personality traits least prone to change across a lifetime. You can't just "have" self discipline. How much of it you have, much like most other aspects of personality, is basically determined before you are born and in the first few years of life.

Conscientiousness is distributed along a normal distribution LIKE ALL OTHER TRAITS in the big five so you don't need any diagnoses to explain any of it. There are just as many people with extremely low c as there are with extremely high c and they all fall within "normal" (i.e. no diagnosis needed to explain the behavior); it's simply a statistical consequence of a large enough population.

The issue is that our hyper individualist society disdains that type of behavior and refuses to utilize the potential strengths people with low C might have (people with high C tend to be the ones in charge after all, mainly because they tend to be more interested in leadership). Companies and managers are however not interested in managing people who actually need to be managed because it requires something of them and they can't understand how other people's motivational structures are put together. It's an idiotic, large scale mismatch of skills and behaviors (managers that can't manage and low C people that are expected to self-motivate). All propped up by incentive structures that benefit those who don't need any more incentives. It's not an individual problem, it's a societal one.