I have everything going for me, but no confidence... I need help! How do I believe in myself?

Hi I'm 16 and I have everything going for me. I am about to become a senior. I am top ten in my class. I have a 4.0 gpa. I take ap classes. I am a member of student council, the leos club, and national honors society. I am president of student council and national honor society. I play three varsity sports. I've been captain of my rec softball team since I was a sophomore and I'm going to be captain of my school soccer, basketball, and softball team. I made all conference 2nd team for softball my junior year. I have a family that loves and supports me and the most awesome friends a girl could ask for. I'm also pretty popular at my school. And i have a wonderful boyfriend who loves me to death. I work at a summer camp in the summer and basketball camps in the winter. I do community service for my church and school. And I am a very easy going person with a not too shabby sense of humor. But I have a confidence problem. This problem is affecting my life as well as my softball batting average significantly. I need help:( I just tend not to believe people when they say I can do it. I'm also kinda shy and hate admitting I'm wrong. I often become very negative and am unable to convince myself to stay positive. But I don't know what to do in order to believe in myself and therefore have a happier lifestyle. I guess I an afraid of coming off as cocky. But at this point I don't care. I want some confidence!!! And I need some thoughtful phrases or sayings that could help me out? Or maybe just a few life pointers?