Glancing over what you've said about your world, you seem like you're about to fall into the same trap I did. I spent a month mapping a whole city, block-by-block, wrote out encounter lists, loot lists, places of interest, a few NPC ideas, a loose plot for them to follow...I got so focused on making sure I had all my details arranged, I left little room or energy for the spontaneity that is the bread and butter of roleplaying.
The players, dutifully and being extra sensitive to testing my limitations, just kind of followed my plot trail. It wasn't very inspired and when the energy starts going out of a session, you can feel it. Just like what others have said above about the appearance of control, if you start stumbling trying to work up ideas to move things along or fill in the gaps, it destroys everyone's suspension of disbelief, even for fun's sake.
Players can tell when you're phoning in a monster encounter so you have room to think. It's great that you have a map where you can point to any place and generically say something about it. But if you're not prepared to fill out every building in there with different and interesting NPCs, on demand, it becomes a trap for you as a DM. It's presenting them with options you only have mediocre responses to. Knowing that peasants live in this building doesn't actually help you when players show up and expect them to be different than the last set of peasants they met.
Say your setting is a fantastical wizard's tower. You know how hard it is actually coming up with something fantastic every room and vista, that isn't a blatant trope, cliche' or panicked choice? If you write that a room has an anti-gravity sphere in it, players will want to mess with it, and if you've not thought about how to deal with anti-gravity, or players trying to carry the orb around with them to float over castle walls...your game is totally in the hands of players at this point.
Libraries are a classic example of more space than ideas to fill it. See how quickly it takes you to go from specific titles to generalizations when players start looking at books. See how quickly things go crazy or you shut down an idea when a player actually reads a book with an interesting title.
That's why I love table talk as a DM. LOVE_IT. Why? Because players aren't paying attention to YOU. You have time to think "How should the world be reacting to what's going on right now? What's next for the players? Is it time to introduce a side thread/quest/NPC? is it time for a combat of sorts? What's going to be in the next room they explore? What fantastical details should I give them, and should they actually do anything tangible?"
Table talk becomes bad when you're ready to go, you've got your plan or ideas, and the players aren't paying attention or have to be dragged back into it. That's when it's gone on too long. But don't be afraid to let them get a little distracted (as a group, people on their phones texting or tv is the devil when gaming), because it can only help you make the next scene you deliver better.
It's time to think you DON'T have when players say "I open to the door and go into the next room." They're looking right at you and watching your brain work through your facial expressions. If you're not prepared, it gets awkward quickly.
For someone with a large open world that they probably just want people to wander through, trust me, the pace at which you have to create whole NPCs and quests ideas is demanding. Going from a large, chaotic combat straight back into actual roleplaying is tough, so don't underestimate how much it takes out of you.
Lastly, be ready to accept outcomes and react to them. I've made DOZENS of super monsters that players just steam-rolled in 1 or 2 turns of combat. Utterly steam-rolled. As a DM, that hurts and if you're not good about managing your emotions, if quickly shows to players. You put a lot of love into that monster, and the players didn't really appreciate any of it. Be ready to have that situation happen every. single. encounter. Don't get attached to NPCs and what eventually happens to them.
Be ready to adapt, even cheat, if you have to. It's all about having a dramatic experience. Don't change their Armor Class every round, or make them automatically hit, but if they need an extra 20 HP so they get a turn to get away and fight another day, then do it. If they suddenly have a magical teleportation amulet or spell, do it.
That doesn't mean your villains should never die. They should die after a good fight/challenge. You should do whatever you can as the DM to ensure that happens. If you have a guy you don't want dead now, be ready to think up ways and reasons he's not dead or got away that don't piss your players off. A good one is the Rod of [insert what you need here]. Things going bad? Baddie already at -20 HP in the 2nd round of combat? Well he pulls out a rod of Fireball and blasts it in the vicinity of the players. Walls collapse. Players are thrown off their feet, possibly horribly injured. Bad guy limps away through the smoke and fire.
Just never take it personally. It's so much harder to go on as a DM if you feel like you've failed, or are failing. Keep an open mind about outcomes and stay mentally agile. Stay positive. The worst tests of your DMing will be players who just thrown themselves into encounters because they're not concerned about the results.
That turned into a novel, so here's some TLDR;
-Extensive preparation is excellent. It is only 1/2 the battle though.
-Mental agility is the other half.
-Start a list of one sentence ideas/phrases. Ex:
"A grieving widow."
"A young boy eager to become an adventurer."
"A tavern that is a front for something unusual."
"Giant Eagles attack."
"A golem that became sentient and is fleeing its master."
-Glance at this list during game frequently when you're planning ahead for what you want to do.
-Don't take "losses" personally.
-Adapt to your losses.
-Cheat, if you have to, in the name of a better game. But be subtle about it.
-"The best laid battle plans are meaningless when the battle starts." Or something. It totally applies to DMing.
-Let table talk happen if you need time to think.
-Never lose your sense of fun.
-To me a laptop is a must. Being able to tab between written pages or graphical documents keeps things moving. Book, notepad or note card flipping is blech. I'll even write out full NPCs on their own pages so I can reference a ton of information on them quickly.
-Be a Zen DM. Compete, without competing.
-Learn to spot problem players. As a troper, you should recognize someone that is playing your game that way. Don't get ruffled by how they plough through encounters like assholes, effectively daring you to do something to them. Don't bother. Killing them will just reinforce what they already believed; that they knew how this was going to turn out.