With any significant goal you would have invested energy, time, emotions and physical effort. Once people complete the goal it is not uncommon for them to feel a sense of loss. It may not be as acute as LeBron James above but I remember completing the global rollout of a HR technology platform and was actually quite sad as a result.
What will I do with my time and now I have done something this big as that what else is there to do?
The remedy is to always have your next goal in mind. As you come to the right point in the present goal you start to redirect energy onto new goals. This is how successful people maintain their energy and resilience.
When elite sports people achieve their goals or winning the Premier League, an Olympic gold medal or break into their national team do they stop there? They always have something else to aim for and we should think no differently.
When you look at your goals think about what you will redirect your energy onto when you accomplish the goal.