When I am running a game, I am never as prepared as I really would like to be. It is a rarity when I actually have all maps drawn out, encounters totally envisioned, and a course of events totally plotted for the story I am attempting to tell.
This is not to say that I go into something totally blind, at least not most of the time. For a few years, we had a group that used to play every Wednesday, every week. More often then not, I was the GM for a group that could be as large as 6 to 8 players. The great thing is that some of the players created intricate backgrounds that at times put the players against each other, at least in regards to some agendas, nothing that directly caused any outright division or escalated confrontations. It was a Neutral-to-Evil themed game in a city filled with assassins guilds, thieves guilds, and feuding families. More often than not, the players had an idea and I was able to go with the flow and create around that. Looking back on the notes from those adventures, they were really some of the best games I have ever been a part of.
Many times, I run a session and I know that I missed a specific item that I wanted to deliver, or I did not time a certain event the way I really wanted to or intended to. There are also times when I finish and I think that this one thing I did really ruined the game, or at least made it so it was less enjoyable for the players.
GM Reminder #1 - Players do not know that
The players are more than likely still having a good time and you may be a bit hard on yourself, or that is how I think at least. Remind yourself that as far as the players were concerned, the events happened as they were supposed to. Maybe, it actually turned out better that way.
I know I am guilty of coming clean after the game and letting the players know that I wanted to do this or should have done that. I need to continue to remind myself that I should not do that. If the session seemed to be successful, or at least the players all seemed to be engaged and interested, it was probably just fine.