perserverance

Why is it that I seem to lose steam with every new thing that I try? I have this great idea for a story to write and I’m excited about it only for a few days. I want to learn to play the guitar and yet I stop going to the classes halfway through. Why can’t I even go to a one-hour class for six weeks? And, then I get this great idea for a new career and I look into it for only a day or two. What instills perseverance in a person? And why don’t I have it?