My senior year of high school basketball.
I've played basketball for as long as I can remember. I was always above average at the game, but never a stand out amazing player. But, I was looking forward to basketball my senior year more than anything. We had a pretty good team that year, and I was one of two seniors on the team. It was the day before our 4th game of the season, and out of nowhere I get this terrible back pain. I think nothing of it, sleep on it, wake up ready to play the next day.
Game starts, I realize instantly I physically cannot run without a pain that is bringing me to tears. Coach sits me out because he can obviously tell I'm in pain. I go to the doctor, they originally say it should be a, at max, two week problem, and should go away fairly quickly.
Turns out to be a season long injury. I don't get to practice or play in a game the rest of the season, except for our last home game of the year. Doctor didn't want me to play, but there was no way in hell I wasn't going to play one last time. I played for about 4 minutes that game, and I cherished every moment of it. After the game, I was an emotional wreck because I knew it was the last time I would ever play basketball competitively for school.
Being injured made me realize how much I loved basketball. I would of done anything to just be out on the floor running sprints and doing drills during practice with my teammates. I was an emotional wreck during the entire season. I went through some mild depression, but I learned a lot from it. Typically people learn "You don't know what you have until what you have is gone" from losing a person, but I learned it from basketball. I also learned that you can always try harder, no matter what it is. School, sports, your job, you can ALWAYS try harder.
tl;dr: Injury during high school basketball taught me you can always try harder and that basketball was more than just a game to me.