One reason why the transition between professional and athlete status is so challenging is because of the demands of "the next level."
When an athlete moves to a "professional" status, it represents the highest level in their craft. All of their endeavors have led to this particular point. The demands of the professional level of sports reflects this intensity with longer seasons. Even when they are not in competition, professional athletes are involved in training and detailed working on their craft. With the emergence of analytic data, there is much more that athletes have to do in the offseason. Such demands involve examining prior performance over the season, dissecting bio-mechanical approaches to the game, and ensuring that they are buffering their own areas of slight and increasing awareness of their opposition.
When an athlete is a professional, they are competing against other professionals. This is another reason why the demands of the professional level is so difficult. Athletes are no longer competing against "kids." Rather, the competition at the professional level is grown men and women who view their job as the particular sport. There is a greater focus and drive in professional athletes because they have made the sport their job. It is their only focus. It is far from a "game." It is work. Failure to be the best they can be means they lose their primary source of income. This drive is another reason why the transition to the professional level of sports is challenging for amateur athletes.