This might help. Almost all the masters runners I know have a pre-race ritualized warm up.
I don't race 5Ks any more, but when I did, I would show up xx minutes before the start, pick up my bib, hit the porta potty, and do some easy jogging. For me, the jogging started 45 minutes before the race. After a mile or so, I'd go to the finish line and run the course backwards to pick out the point I would start my kick. I'd then jog back to the start, feeling fresh and powerful so I could recall that feeling when I reached that point in the race. About 20 minutes from the gun, I'd start my drills: lunges, leg swings, karaoke, fast knees, and a few strides building up to race pace. At ten minutes to gun, I'd switch to race flats and do a few strides until I got the lacing just right. I'd get to the start with a couple of minutes to spare. If possible I'd do a stride out right from the start for 30m or so. I also had a plan for the first 30 seconds or so of the race, but I won't go into that.
Having a pre-race ritual will reduce your anxiety, but if you get nervous, remember that nervousness means that your body has turned on the "on" switch to race, but since you're not racing yet, it has to manifest some other way. You perceive it as being anxious. You need to switch your mindset to realizing your body is saying, "I'M READY NOW!!!"
Better to be a little nervous than to not be nervous at all.