A Style or Category More Than a Tool

There is a style of games in which the GM gets to be surprised and explore unknowns. These are games where the players also have the right to add story details, places and NPCs to the game world, and otherwise take over parts of the usual GM's repertoire of powers.

In some games, such power is highly restricted and/or regulated. In others, the line between the GM's and the players' ability to directly shape the world by fiat is much blurrier.

These games are known by a plethora of names such as 'collaborative', 'narrative', 'storygames' and perhaps some others. There is surprisingly much flame-warring about what exactly 'narrative' means what are minimum requirements for a game to use such adjectives. But that's not as important to this question as understanding what games should help with your problem: games where the GM gets to explore parts of the world created by players in a manner similar to how players explore the parts made by the GM.

Examples of such games:

• 
  Microscope. A 'high-end' example, to the point of being disputed as an RPG, where participants get to define huge parts of the world, and are encouraged to just add a world-feature on top of whatever's been added by the previous participants. E.g. one may say that the galaxy includes aliens, but another adds the twist that meaningful communication with them isn't normally possible. Keep in mind that Microscope has very weak attachment to characters - probably even less than assumed in 'dynastic' playstyles!


• 
  Various FATE games. A 'mid-range' example: players do have ability to shape the world, but the ability to do so depends on expending limited resources and/or successful rolls, and there is some veto power wielded by the GM.


• There are many normally 'traditional' games (i.e. non-storygames) that add 'low-end' elements of such player abilities. In them, such powers are highly limited in scope, magnitude or both, and may also be very resource-limited. Such as GURPS (a game with an overwhelmingly 'physics-oriented' reputation!) allowing players to give their PCs lucky coincidences if they have the (expensive and limited in use per session) Serendipity advantage, or Impulse Points (likewise moderately expensive and slowly-regenerating). Or Exalted allowing players to add small details to the world if they make a sufficiently glorious description of a character's action that already builds upon world details that have been described by the GM.


• 
  Apocalypse World, Dungeon World, and their derivatives instruct and require the GM to play to find out what happens, and to not plan ahead too far (the system fights you if you try). They support improvisation and exploration in the moment without having to define things thoroughly ahead of time. That probably puts it on the a 'mid-to-high' part of the spectrum.


• 
  Fiasco. The GMless, story-focused nature of the game puts it on the 'high end' of the spectrum, though not as high as Microscope because it is more attached to actual characters by comparison.

The Cost of Discovery

Of course, such a style is not necessarily for everyone, and not just in terms of being not to everyone's taste. GMing in such a style requires being more ready and willing to improvise, to tolerate carefully set up plans being invalidated by a seemingly-inconsequential detail that turns out to significantly change the context and the like. It can result in the game world ending up less coherent under the weight of all the additions - more Star Wars Expanded Universe than Historical WWII. If you and your players are fine with that - great!

Don't invent and plan your entire world in advance.

It's always a good idea to have a basic plan ahead of time. Knowing that your players will start in a city and you want to throw a hook at them dealing with a crime boss (maybe they get mugged in an alley to get the ball rolling?). But you don't need to plan out the entire world. Sure, they're in a city, and the city has all the things that cities have such as shops, inns, bars, etc. So wait for your player say that they want to go to the shop to try to buy an item. On the fly, make up the shop's name, make up the shopkeep. Create their personality as you go, in response to what the players do and say. This way, the experience is new to you too; you're also discovering the shop and learning about the shopkeep.

Now, if you do do this, there are a few things that you'll have to do as well:

1. Keep notes! You've now made Fizz Wizzets Magical Gadgets, and Fizz Wizzet is the owner, a gnome with red hair and a bubbly personality. Write all that down so that the players have a consistent experience next time they want to visit the shop.


2. Have a very basic outline of ideas. Plot hooks, key story points, major characters and their motivations, etc. It might also be helpful to have a list of possible character name and personality traits that you can throw together at a moment's notice to create someone new as needed.


3. Be good at improvisation. Good DMs can always improvise on the fly a little bit, but to be able to run a game like this, you really have to do well. You don't want to drag the game to a halt while you try to make up a name and decide how an NPC will respond, you need to try to keep things moving whenever possible.

This can be done with many TTRPG games, including D&D and Pathfinder; but some games are better than others. Further, there are some games that help to offload some of the work onto your players; see other answers for examples and details.

Personally, the hardest part about this strategy is combat encounters. It's one thing to make up NPCs and improvise their reactions and responses. It's another to make up a combat encounter on the fly and have it be fair and balanced; especially for a less-experienced DM. I would advise planning out a few combat encounters in advance. You can shift around the name and description of the enemies, but have an idea of how many there are, what their statblocks are, what weapons/magic they have, etc. When it makes sense to put your players in combat, you can then decide if they're fighting Human Thugs, Orc Raiders, Kobold Guards, or Royal Knights. At that point, you're just putting a skin on the block of stats and abilities you pre-prepared.

Put that exploring energy into worldbuilding

Explore the world, but instead of it being another DM's world it is your world. Let your imagination fill in the pieces as you construct the world.

In my campaigns I start with a single town or city, often I take this from a module or adventure to make things simpler to start. Once I need to expand the world I explore it as though I were an adventurer.

An example of the train of thought this leads to:

What happens if I head north from here? Maybe there's mountains.

How do I get through the mountain? With a little searching there is a mountain pass.

Who controls the pass? The pass is guarded by Hill Dwarfs who won't allow passage to outsiders.

How do I placate the dwarfs? If someone were to deal with the nearby Winter Wolves the Dwarfs might let you through.

What lies at the other end of the pass?

Worldbuilding like this is still exploring, but exploring your imagination instead of a pre-made world. To me that's more exciting because there are infinite possibilities of what is around the corner or over the hill.

Additionally this gets me more excited for my players to explore. When they explore I get to share my discoveries with them. This makes me a more engaged and (in my opinion) better DM.

I also enjoy exploring, and I would encourage you not to underestimate how much exploring your players will cause. If you create a robust world and give your players freedom to do some exploring of their own, they will almost certainly go off in directions you didn't expect, and require some improvisation. Even if they don't go off in odd directions, you may find that watching others enjoy exploring the world you built brings you some joy. If the idea of bringing your carefully built world to others does not sound like it would be enjoyable, then DND may not be for you. However, suggestion or shopping posts are not really within the scope of this board, especially once we depart from DND and the wide world of tabletop RPGs opens up to be explored.

You can absolutely still DM! You can get the same enjoyment of exploring out of being the DM as being a player. I DM and I am often surprised at what the players in the party do and how it changes the story. I get a great deal of enjoyment and exploration out of preparing ahead of time before we play.

Get into the lore and explore things like the Sword Coast Adventurer's Guide. These can give you a sense of adventure and immersion as you go along.

As others have also answered, definitely check out some of the pre-made campaigns like the Lost Mine of Phandelver Starter Set campaign. This has a story, world, and villains already made up that you get to discover and explore along the way with your party.

You can also explore some of the tutorials on Improvising a campaign. This lets the story grow and unfold in ways that you wouldn't even expect and allows you to get enjoyment out of not knowing what is going to happen next.

Bottom line is just have fun!

As a volunteer-to-DM myself, I often feel that too. I really enjoy both DM and PC sides but at first, I'm a DM because I couldn't find a DM. So here are two pieces of advice:

Build over the pre-built, and improvise

As Linksassin said, you can use your imagination and exploration craving to build you own stories. I would suggest to begin with pre-built stories, then you'll quickly start to add your own paths and forks. Then, when you'll be more familiar with the game you play at, you can build your stories from scratch. I'm sure you will truly enjoy to see your PC explore your stories.

Switch sides!

In my current group, I was lucky to have one of my players who told me he wanted to try DMing too. I jumped on that occasion since I admitted that I missed being a player, just exploring and improvising my way out. So we agreed, with the group blessing, to switch sides at the end of every scenario. We just have to find some RP justifications (He plays a priest that have to retreat in his cult's temple every now and then, and I want to play an semi-hermit druid, so that was easy). So you could suggest that to your group, and maybe one of your player will understand your situation and agree to switch side with you.