I ended up in a small twitter conversation the other day. Philgamer mentioned me as someone he'd love to have in an L5R game (so honored by that :D ) but mentioned anxiety being a problem - along with conflicting schedules. The other people tagged all chimed in fairly quickly, and turns out we all suffer varying degrees of anxiety around our GMing. It's fairly normal it seems from other conversations I've had.
How it happens is easy to see. If you're serious about running not just a good game, but the best game you can, then there is a lot of pressure to perform. This gets even bigger when you have a good session - or campaign - and find the next one not quite living up to those same lofty standards.
The trick is, some people get so anxious about this - about needing to make their game better and more fun for everyone - that two things happen: First, the GM doesn't have fun running the game. Second, the game suffers because of this.
Trust me, if you're not having fun running your game, your players will pick up on it and it will curb their enjoyment and enthusiasm as well. Fortunately, there's an easy solution - easy to say that is, anyhow.
Relax, and enjoy your game.
Yep, that easy. The harder version is accept that some sessions will be better than others and that's alright. Some nights your players will be tearing to go, and other nights they might be a bit more lethargic about it. The same is true for you. Some games will be better than others. Some campaigns will be better than others. That's fine. But don't get so worked up trying to make things amazing that you miss all the awesome going on in the game right in front of you.
Find the parts of your PCs you love, and play into them. Do things you like in the game to mix it up. Have fun with your game. Otherwise, what's the point?