Sometimes, a GM who has been at the game a while its a creative or emotional break. The following advice may be of use for recapturing enthusiasm and growth.
Play with other GMs[]
Veteran GMs are often saddled with the syndrome of always the GM, never the player. Play in someone else's game once in a while, even if only occasionally. Organized play is a good way to find a game where you can simply play, without the pressures of GMing, or just the social pressures of your regular group in general.
Homebrew something[]
Make something cool for a player who has been looking for a way to engage in a theme or concept for their character. Surprise everyone with a new monster. Create a new trait or piece of equipment. Start small, but follow your creative mind.
Use your tools[]
Just because you have the ability to create new worlds and systems from whole cloth doesn't mean you always have to. You've done this so many times, you can throw together something in a few minutes less experienced GMs would agonize over for days. Recycle plots. Recycle NPCs. Effortlessly thumb through your books for a monster for every occasion. Most of the interesting stuff in a game happens in session. You have the skill to turn an ember into a flame. There is no shame in doing what you know, and doing it well.
New players to your group will think you make it all look easy.
Try something different[]
Play other games, even ones you doubt you would pick as a favorite. At worst, you will be reminded what you love about your favorite systems. At best, you will unlock a fun new interest.
Learn a new skill[]
Practice a character voice based on a real accent. Make some terrain. Play with descriptions. Relax and talk like a normal person. Whatever you are doing, a little stretch can be refreshing.
Take a break if you need to[]
Seriously. For any reason, or no reason. Games are supposed to be fun.
Set limits[]
You have the right to say a weekly game is too much... or a monthly game is too much time between sessions. Just because you have always gamed with someone does not mean you have to tolerate negative behavior. You and your players may have work schedules that need to be honored.