No matter who you are, elite, professional, top age-grouper, masters athlete, mid-packer or back-of-the-packer, if you have high goals then you will be nervous before a race.
So many athletes fear being nervous, as if it means something is wrong. Nothing is wrong. Being nervous simply means the goals you have matter to you, and the day has come to see if you can accomplish it. Many think the best athletes don't get nervous. Trust me, they do. They just tend to handle it better than most athletes.
Think back to your college days, and heading into a major exam. You were probably nervous a lot of the time, because there was a lot of the unknown, and you were likely paying for that class too! The cost of failing was not good. But in the end, you likely knew the material or didn't. You were prepared, or you weren't.
Don't fear your nerves. Embrace them, and use the arousal to get the adrenaline pumping and producing a great result. This graph illustrates how the right amount of nervousness and arousal can help your race performance:
If you're not nervous, then you have low energy level, and likely will have a poor performance. Think back to races where you really didn't care about them, or weren't excited. They likely were not your best performances.
If you let your nervousness overtake you, it's too high. You begin to doubt your preparation, your plan, your diet, anything and everything you make a stressful situation.
Understand and accept nervousness, not taking it as a negative sign, such as a lack of confidence or preparation. Trust in your preparation, and execute your race plan.