How to build one's confidence?

I used to be a self-assured girl when I was in the middle school.

In that time, I was good at my study, and all the people are friendly to me, especially the teachers. Life was full of sunshine in those days. Althought the learning was hard, but I enjoyed. 

I used to be the leader of the girls in my class, and was active in the school life.

However, my sunshine life have faded away since I entered the University. And last until  now.