One possible version:
Years of school life has taught me a lot of things, of which teamwork is the most important for me.
In fact, I didn't realize its importance until I was chosen monitor of my class in my senior middle
school. At first, I did most of the duties myself while other students care little about class activities. As
a result, I was tired out and depressed. Then I turned to my teacher and he advised me to cooperate with
others. Thus I began to recognize the strengths of my classmates and have everyone do their part in class.
It is working in teams instead of on my own that has freed me of trouble and made my work more
efficiently.