DMing is the sort of activity that really requires you to be on your toes. You have to react to what the players do, as you craft the world and the adventure around them. I'm generally pretty good at this. As evidence, I've been running the same campaign at my FLGS for over 4 months now with no real prep. I usually don't even think about what the night's session will be like until I'm walking to the store.
That works for me because I've got decades of experience as a DM, I've absorbed a whole lot of stuff from outside of D&D that I can throw in, and I try to make sure that my references aren't so esoteric that no one gets them (most of the time). Players love getting the joke/hint/reference. I also don't get super attached to either my NPCs or plot lines.
But my players are getting better, and their characters leveling up... And honestly, they know the 5e rules better than I do. Not a bad thing, but it does mean that I need to up my game. I've gotten caught a little flat footed a few times, and had to stop game to grab monster stats and figure out the next steps.
So going forward, I'm going to try to be a little more prepared, and have the weeks likely monsters bookmarked/note carded for quicker action, and a few of the likely needed plot points fleshed out. We'll see how I do. Last week went pretty well. I had the monsters that I figured I'd use, along with the spells one of them would use. One thing I didn't count on was the paladin. He's been with the group pretty much since the start, but mostly he's just been a beat stick. This week, he discovered a whole bunch of abilities he's never used, which resulted in the death of the evil wizard thing. I'd kind of expected that the evil wizard would kick around for a little longer. Ah well. At least I've still got another day to figure out what'll happen next. Probably something bad with the demons he's been summoning.
In other developments, one of my players is thinking about switching to the other side of the screen, and while it means I'll lose her at my table, some other players are going to have a great time. We were talking after game about how running a table is like being a stage director, and I added "where none of the actors even bothered to read the script." It got a laugh, but it's true. Once you're at the table, and the scene begins, that's it. You've got to let go of any script that you developed, otherwise you'll be frustrated with your players, and no one will have any fun. Most of the players are the actors, the DM is the directors and re-actor. The DM sets the world, brings it to life, and drops the PCs into it. Then it's up to you to know your world, whether you've got everything written out (what a waste of time) or just doing it instinctively (takes practice).