I very rarely get nervous. I've found that the best cure for nerves is to prepare. Then, even if things don't go as planned, I know that I did all that I could leading up to the event.
Well, I am signed up for a 10K on Sunday and I am nervous.
Recently, my runs have sucked and I don't know why. There are legitimate factors like the heat, but I'm starting to wonder if I'm just in worse shape than I thought. I look back on my training log and before I got sick, I was cranking out awesome bricks... 15 mile bike rides followed by 6-8 mile runs at tempo pace. I'd been doing this weekly for about two months in addition to running three other times a week. But since I've been sick I've been having a hard time cranking out 6-8 mile runs at a slower pace and without the biking beforehand. I've been consistently hitting my other runs and those have been going great, but my long runs are really lacking. I can't figure out what is going on and I'm not sure what this is going to mean for my race on Sunday.
I also think some of this is psychological. This race last year was my first 10K in about a decade. Going in I was dehydrated, I rolled my ankle getting into the car the morning of, and I had no idea how to pace myself or what I was capable of. Even with all of that I far exceeded my expectations and had what, for me, was a really great race. Then in September, after a summer of marathon training, I smashed that solid performance with a 10K that I still can't believe I ran as fast as I did. That leaves me with a most recent 10K that is well out of my reach and a prior strong performance on this course... I have a lot to live up to with this race which compounds a bad two weeks of running.
I put off registering for this race all week but then I realized that NOT running this race because I was scared of doing poorly would be taking the easy way out and the wrong reason for not running. I'm not that person, so this morning I signed up. If nothing else, even if I have a bad race, I'm really glad I'm going to get out there and give it a go. Wish me and my nerves good luck 
Good luck Kat! I hope it goes well but don't worry about if it doesn't, its not the end of the world and there are certainly other 10Ks out there for you to crush when the time is right. You know, you said your regular runs were going well but the longer run was what has changed and hasn't been going as well, right? The fact that its hotter out too makes me think there might be a nutrition solution for this road block... caloric and/or electrolyte-related perhaps. Can you break down what your intake looks like before/during/after these runs?