My take on this: Time for a break. Maybe even a new group.
In my own experience, I find that I get the most frustrated when I switching from GMing to storytelling. When I have epic plots and a plethora of devious NPCs that are going to do The Big Bad Thing on their schedule and damn the PCs. In that situation, the PCs aren't actually players, they're actors in a script I've written - except I haven't handed them the script so they get off script and I get ticked off. When I realize I've made that mistake, I apologize and move on.
For example, in my current game I've worked up an incredibly complicated plot. A tiny kingdom exists between two large empires, one human and one gnoll/human. The human empire, which would normally protect the kingdom, has thrown all of it's resources into helping mountain dwarves fight some unknown threat from the underdark. So now, the little tiny kingdom is feeling a little less confident. The king dies unexpectedly, and now an eleven year old girl holds the throne.
Sensing weakness in the capital a number of opportunistic barons are making plans to take out the young Queen, they are led by her supposedly loyal cousin - who is in fact deep in debt to the elven Royal Houses, and they plan on putting him in power so they can gain their own port (no one is really good in any world I create btw).
So the players are in a port city, one of the biggest in the kingdom. I want them to meet up with the head of Her Majesty's spy ring, the Black Rose. The Black Rose is aware of the threat but they are afraid to move openly due to how uncertain the whole situation is. My plan was to have the players work with the Black Roses and get involved in some pretty deep political and skullduggery type stuff.
But what happened was slightly different.
The players start to explore the city and find the market quickly enough. And they ask what's going on. And so I describe general market stuff, and on the fly say "Oh and there is a recruiter station. The local Lord General is attempting to recruit mercenaries." So they want to go talk. And I'm now making stuff up on the fly. So I say that mercenaries are being recruited to reinforce small forts along the southern border facing the gnolls. In addition, they are also scouting old ruins for potential use as bases of operation.
So they decide to take a mission.
Now I've made up a whole secondary mission of going out to this old manor house to explore it and find some threat and some treasure and have them report back where the recruiters will refer them to the head of the Black Roses.
So yeah, two or three sessions shot - and I'm totally making crap up on the fly for the games, but what the heck? Why not?