I need motivation

When it comes to school and work, I have the motivation I need. I wake up when I need to, turn in papers on time (if not early), always show up to my shifts on time....blah blah blah. But when it comes to my personal life, I seem to have none. Why is this? You would think it would be the opposite. You'd think that in my personal life I'd just do what I want. I set many goals for myself, both personally and professionally. One of those goals is to write a novel. It's something I've always wanted to do and I have what I consider to be a great story in my head.

Here's my problem. I never write! I get so excited and tell myself "I'm going to sit down tonight and start on this". And I never do. It's not like I can't write, I consider myself to be a decent writer, I just don't do it. And it's something that excites me. The counselor in me wants to say that I avoid it as a way to protect myself. If it turns out really bad, I'll be upset. If I can't get the ideas to flow smoothly from my head to the paper (or computer screen), I'll be upset. So I just don't do it.

There's another reason why I'm afraid to start writing it: I think it's a really great story. Every time in the past that I've taken an idea and put it onto paper, it turns into a short story because I get impatient and wrap it up quickly. Which makes the story not deep at all. I don't want that to happen to this one, because I like it so much and can see so many branches and complexities to the story. So what exactly do I do? I don't want it to turn into just another short story. I want the great American novel. I want to fully explore my ideas and make it a successful attempt. I just don't know how to make myself start it.