William,
We all have bad spells from time to time. They keep us honest. Without them, how can we truly appreciate the good days or even recognize them? But, as Louis said, prolonged slumps often have a lot to do with confidence. Ironically, sometimes overconfidence and expectation can initiate the cycle of a slump. Expectation of success can cause us to become careless about many of the fundamentals that combine to create success--all of the things that Mark mentions above.
I don't know if this will help, but I'll tell you about a fishing friend who is a true master of the self-inflicted slump. It usually goes something like this:
When flies or techniques that he expects to work don't work, or don't work as easily or quickly as he expects, he slowly begins to lose focus. It is almost imperceptible at first, but once started, it snowballs with every fishless cast. He becomes impatient. He rushes his casts, and his casting deteriorates. He begins to slap ill-timed and ill-considered deliveries randomly onto the water. Often, when he recognizes that he has just delivered an abomination, he rips it from the water in frustration and angrily delivers another. The next cast is often worse, but even if it is not, any self-respecting trout within range is now cowering under a rock.
By now he realizes that he has blown his chances at this spot, so he moves to another. But his approach to the new spot is hasty, motivated by impatience and frustration. He stumbles. He splashes. He curses. He slaps more thoughtless and random casts at the water. Sometimes he'll change his fly, but the change is more from desperation than observation of a need for the change. And when that fly doesn't produce results in a few casts, the casting becomes even more frenzied and pointless. Saddest of all, should some incautious trout respond to this careless flailing, he is not ready to respond and misses the strike....
It is painful to watch, and it sucks the fun out of fly fishing as surely as a party of drunken tubers floating through your favorite hole at the peak of the hatch.
If any of this sounds at all familiar, then you may be experiencing a self-inflicted slump. If so, I sympathize, as I do when I see it happen to my friend. And I also empathize, because it has happened to me.
Take Mark's advice. Slow down. Observe. Select trusted flies with purpose and confidence. Attend to your approach like a stalking heron. Put all of your skill into every presentation. Do not expect success, but be ready for it at every moment of the drift. If you can fish with the same focus on the umpteenth fishless cast as you do on the first hopeful cast of the day, I'm confident that your slump will end.
If not, as Casey says, there's always next year. Personally, I'm getting a bit too old to count on that as much as I used to, so I'd give Mark's advice a determined try first! :)
Best,
Lloyd