There is a lot to be said for consistency. Great daily effort, inspired and driven by a solid plan of action, usually produces great results. Greatness is a journey not a destination and some days the journey is a tough one filled with pain, frustration and despair. I guarantee you that any top athlete, actor or musician our society reveres has had many of these "investment" days along their path to fame and fortune.
For your athlete the process is the same. The legendary NFL coach Vince Lombardi once said, "Being great is not a sometimes thing...it's an every day thing!" The seeds of greatness are planted every day and sprout in due time. Your athlete needs to show up and give her all every time she steps on the field without regard to how great she plays on any given Saturday or Sunday. The results of her efforts will arrive on cue.
As an adult you need to remind your athlete just how far he has come in playing the game over the years. I marvel watching the young ladies play at the high school and 16u travel level that I first coached when they were 6 and 7 years old little girls. To an outsider like me their hard work has definitely paid off. But through the eyes of a child (or teen) progress can never happen swift enough!
Greatness isn't built overnight. It requires years of patient and persistent training, both physical and mental in order to attain any level of mastery. Watching that quarterback throw the flawless 40 yard crossing route pass or the actress mesmerize you with her stunning theatrical performance are products of years of blood, sweat and tears...a continual commitment to the process of mastery.
So on those days when your athlete is frustrated with his performance and wonders if he'll ever "get there" remind him of how far he has already come on his journey and that anything worth having is worth the time and effort it takes to achieve it!