Okay, here are some things you can do...this first one is counter intuitive advice, but it works...
When you are feeling scared and nervous -- try and make yourself even more scared and nervous, focus on the scariest thing, dial it up as high as you can get it, and then make it just absurd.
Two things happen: One, your mind won't let you intentionally scare yourself, so it will actually knock the feelings down a notch or two...and send them in the other direction of just being cool about it all.
If you find yourself cycling back to where you started, just repeat the process. This is a technique that is taught to people who train to speak in front of thousands of people, and it works!
You can also reframe the race as fun. You can even go so far as to imagine your self running in a clown suit and making everyone crack up. See your self running in a crazy costume and laughing as you see everyone crack up and be entertained by your stunt.
These are simple techniques that work. It doesn't even matter if you know the "secrets" behind them. They work.
You've heard of "positive visualization for sports performance" yes? There are tons of studies about having two sets of people training to say, throw free-throws or something...where one group actually throws free throws every day for a month, and another group stands and visualizes themselves throwing perfect free throws every day for a month, and at the end of the month the two groups compete...and the visualizing group won. Top athletes have been using these techniques for decades.
Visualize yourself, in each stage of your race, implementing your race strategy, smooth form, making a move at the right mile marker, whatver - but when you do this, remember how it felt to run well in your best race ever. What you are doing is training your neurology to associate past success and good feelings with success and positive association to this next race.
Here is what is going on when you are being scared and nervous the night before a race, it is because you are associating how well you do in the race with your self worth. Your fear is essentially a fear of failure...because you have so much consciously and unconsciously placed at stake on the race.
Most of that is probably emotional, what some call "sub-conscious." What you can also do to put the race in perspective, is to do some journaling on what your expectations of the race are... Are you making too much of it? What's realistic? Can the race be fun? The act of journaling helps clarify the thoughts and get the feelings out in the form of taking action.
Here's another way to do a positive visualization:
Remember a time in your life when you were having a great time, happy, laughing it up, celebrating - fill your mind and body with the thoughts and feelings of that time, and then visualize your race. What you are doing is associating a past happy experience, with the coming race. Hopefully you've run at the location of where this race is...so you can visualize the course and the race etc.
Any time you find yourself too scared and nervous, get back on the beam of doing some of these things, eventually you will find some things that work for you.
One more thing you can try. And this is a great and effective technique that goes back thousands of years in India, I used to have the name for the meditation, but don't have it handy. It is really simple.
What you do, when you are feeling scared and nervous, or feeling any feeling that you don't want to feel or that you want to feel less...is close your eyes, and try to locate the point of highest intensity of that feeling in your body. It may be the size of a tennis ball, or a soft ball, or a golf ball. Everyone is different...some people hold their stress or nerves in their shoulders, others in their neck, stomach...wherever...I even had a friend that got nervous "stomache" in her...calves, you just never know.
Anyway, find the place for you, once you have located the place where those emotions that you want to calm down are located...find the center point of highest intensity of those feelings...and just keep putting your focus on that center point, your mind or thoughts may drift, that is okay, just keep coming back to the center point...all the while breathing, relaxing into it. Most people find that the feelings "release, relax and are significantly reduced.
Hope some of this helps. And remember a little nervousness is good, it helps you focus, but remember anything over the line of a little nervousness -- does not serve your purpose of running well.
Also, I agree with the person about running a few miles hours before your race... I think it is better to save it for your warm up. I had a friend in high school that used to get nervous and scared like you. He always felt better after races. So he said he had an idea, which was to run a race before a race, and then go to the race. Well, I knew the reason he felt better after races, was because the stress was gone, there was no impending race to get nervous about. Nonetheless he tried the race before the race thing. Went to the race...and then totally bombed. It was actually pretty funny, he learned a lesson.
What you could do, is take a warm, but not hot bath, and don't stay in too long, just enough to loosen up your muscles a bit.
I used to do this before races, and it always helped.
Best of luck.