Why do I do what I do?

I'm beginning to notice that I have a terrible habit of thinking that I can accomplish everything. I think i've been doing this since middle school. My parents would always tell me that "you can't do everything," and I honestly never believed them. Whether it was being on 3 different sports teams, along with school, work, band, (obviously not much of a social life) I always thought that If I wanted to do something there would be a way to do it.

Unfortunately, when I started college I was still in the same mind set. My very first semester at the University I decided that along with a full class schedule, over 14 credit hours, I wanted to be in the marching band, in the Air Force ROTC, and Arnold Air Society. I started out with a job... but felt that I didn't have time, so I quit 3 weeks after school started. I ultimately paid for my poor choice of priorities, getting a starting GPA of 2.7, worse than I had ever even imagined.
With the start of my third year in college, all the same opportunities have been popping up. The hardest thing I ever had to do was make the decision to not enroll in ROTC, and tell the officers in charge. But I did it... I was on my way to making my life easier.
And then I messed it all up. Why? Why would I willingly make my life miserable? Because I crave knowledge, I crave excitement. When everyone else is standing in the back barely getting by, I'm up in front raising my hand, volunteering, getting noticed. I want those in charge to know who I am and what I can do. But most of all I like proving to myself that I can accomplish whatever I want to do. I guess early on in my life I decided that I wasn't content with the usual, and that when I look back on my life I want to be able to say that I lived it to it's fullest.