I used to have a really big problem with race nerves to the point of not turning up at a race. yes, that bad.
There are several strategies, I find sticking my head in the sand is really useful, what this means in practice is that in the 2 weeks before a race, I refuse to think about the race. That does mean that all my preparation is last minute but so be it, it's better than being on the toilet for 10 days solid beforehand.
Other and probably better techniques include visualisation, imagine yourself running the marathon, feeling strong, moving well. Make it a really positive experience in your head.
Another thing, what's the worst that can happen? You don't actually finish the marathon.. I think about the worst scenario and it's not that bad, life goes on, the world still turns etc...
Good luck and enjoy.