Me too. I cruised through all the way up through high school. When I got to college (UC Berkeley), the first paper I wrote, I got a D minus. Boy was that a wake up call. I had to learn to work a lot harder because the level of competition was so much higher.
A good practice to adopt to help with this: join a gym and work out 2-3 times a week.
Learn through the gym how to enjoy the process more than the milestones, and apply the same mechanic to other aspects of life. What's the next 2% improvement you can make to anything in your life?
But my adult life (I am 28 now) went nowhere precisely for this reason.
As a kid, I could literally skip all classes and homework, then figure the subject "on the spot" during tests, and score high enough to still pass.
This doesn't work as adult, but I don't know how to do it right now...