I'd second the "Microscope" recommendation just to get the cooperative creativity flowing again; I've been reading about it from the suggestion by Joe and it looks fantastic. Then as mentioned in discussion of Microscope, you could run a conventional campaign using an older system in the collaboratively-created history. And I liked the other suggestions about cooperative gaming and reading more widely sound great, too. You could also look into general health advice to perk you up in general -- looking into vitamin D deficiency, looking into iodine deficiency, getting more omega 3s and other good fats, eating more fruits and vegetables, avoiding artificial ingredients especially via home-cooked meals, getting more exercise like via a treadmill workstation, and so on. Humor can help too; find new funny things to read or watch. You might even consider "Laughter Yoga" (perhaps even before each gaming session -- you just start laughing as a group, where seeing others laugh tends to make us want to laugh more). These are all things that have helped me in various ways.
People change over time. Sometimes, when you go back to something that was enjoyable in the past, it is no longer as interesting. For example, there are TV shows I found hilarious in the 1970s (like "When Things Were Rotten") that I probably would not find as interesting now, including because I've changed, the world has changed for topical jokes, and I've also just seen the jokes before. However, I have gotten some of those shows to watch them again with my own kid (although even there, it can be hard to compete with the excitement of faster-paced contemporary offerings my kid has gotten used to). Likewise, for a college we graduated from, even if we re-enrolled as a Freshman again, we can't have things the way the were because everyone else has changed, too and moved on, and we have changed and so the courses and situations might feel boring or inappropriate. And if we go back as a teacher, the roles and situations are different (which might be good or bad).
So, it might just be time for you to try something new that builds on what you've previously done or even moves in a completely different way. For example, you might find you get the same community enjoyment out of learning a musical instrument and joining a band with a different crowd. Or, you might take the crowd you are with and have them all try something playing music together or doing some other activity (the cooperative "Artemis Space Ship Bridge Simulator" perhaps?). Or, you might try just writing works of fiction perhaps, around some important theme; my own favorite theme inspired in part by James P. Hogan's writings: "The biggest challenge of the 21st century is the irony of technologies of abundance in the hands of those thinking in terms of scarcity." Or you could try to write down what you liked best about the old campaigns (including perhaps life lessons), or you could try to turn them into fictional stories. Of those options, you could identify what you liked most about being a GM and emphasize it in a new way -- was it the creative aspect (writing), or the moderating aspect (officiating), or the group aspect (cooperating)? There are plenty of new activities in all three of those areas, although it may be hard to bring them all together the way GM-ing does. One important thing to remember about "burn out" is that you can't burn out unless you still care. :-) But you can examine any frustrations you have with the current situation and use that as advice for moving forward. So, the question is, what is a good way to use that impulse to care for something given your past experiences?
The alternative to doing something radically new may be to come back to the old with new ideas. Cooperative games have been mentioned; you might try some cooperative board games (like from Family Pastimes) for a while just for something different about game mechanics you could bring back to the old games. Many people have never played a cooperative board game. Along those lines, how could you use the old game mechanics to have a dungeon that players would have to cooperate more with, rather than just slay all the dragons? Similar to one of wberry's suggestion, you might take some ideas from the video game "Dungeon Keeper" where the point is to have a well run Dungeon and protect it against "heros"; so, you could go back to an old game system and have the players role-play the monsters and keep up the dungeon, while the NPCs are the "heros" who come in to steal all the gold and need to be defended against or otherwise taught some cooperative manners. That might be a new challenge to create such scenarios while keeping to the old game mechanics. Or, you could try running old campaigns with some challenging new restriction (maybe a funny one, like all utterances need to be in some poetic form like haiku or sonnets etc.). Or, as another alternative, you could try using what you know to teach a new generation of gamers the old techniques, to see the joy in someone for whom it is all new, and maybe have some of that rekindle your interest in the old games as you explain them to others.
BTW, on becoming child-like again (as opposed to child-ish), there is a great poem by Khalil Gibran called "On Children" with the line "You may strive to be like them, but seek not to make them like you. For life goes not backward nor tarries with yesterday."
Good luck moving forward in some interesting way!
