Well, I think most of us get these. Some races more than others ; the greater the pressure, either from yourself or well intentioned family; the worse the nerves can become.
Different people have different ways of managing; and for some, these nerves are a buzz rather than a hindrance.
For me, a few bits and pieces can help. Packing my running kit in advance helps- I would be absolutely stuck if I had forgotten an irreplaceable piece of kit, such as trainers or a sports bra ! Other pieces of kit can be cobbled together - but essentials are essentials. So, pack, recheck, recheck.
Trying to prepare directions in advance, including getting to the race start, also helps in preparation. Parking on race day may prove a nightmare if you haven't sorted a sneaky parking space in advance, and all contributes to a pre race panic.
For food before a race, I try and get down as much cereal and coffee as possible. However, sometimes I need to resort to the option of eating anything which is not going to provoke a nervous vomit - in these instances, I certainly give myself a break from trying to be healthy.
City marathons tend to prompt the most nerves for me, due to the crowds and pre race build up. So, headphones in, favourite music on, - these distractions work.
The most important thing to remember, however, before any race : You are more prepared for this event now, at this point, than you ever have been previously. So go with it.