Finally, one thing a musician can do to find his/her voice is to stop listening to everything for a bit here and there. This is hard to do as musicians use their ear as their most vital tools when expanding their sonic appreciation. This habit, however, can have consequences because musicians get into their own heads. They begin to doubt their own abilities, and worst yet, they begin to compare their abilities to others & wonder why they are not as good. At times of great artistic frustration, you need to depend on yourself the most, and being mired in self-doubt will do you no good.
Technology is all around us, and with that technology comes the inherent need to invest our time & energy into the techno-wizardry that promises to make you stand out from the crowd. There was a time when the technology available to musicians was the introduction of electricity to musical instruments, and these advances led to some of the most celebrated music of all time. Think further into the realm of classical music and opera. Nothing but the pure expression of sound coupled with masterfully composed works were needed. Sure, creativity & ingenuity are also important ways to gain inspiration, but it may be time to take a lesson from this simplicity with your own music at times.
Validation as a musician can be all-encompassing. Just look at the various guitar-based video games that use crowd noise (positive and negative) to push the working musician to new levels of stardom. You need to understand the road to the top is a tough one.
Remember that finding your voice as a musician is, at its most elemental, finding a way to express who you are as a person. Drown out the noise, and trust in yourself.
It can be argued that the most frustrating experience that an up and coming musician can have is being able to define their signature sound/voice. After all, you are just one piece, a cog, in the machine that is the world of music through the ages. No matter how much knowledge you acquire or how much time to invest in fine-tuning your craft, you sometimes you feel as though its all for nothing at all. A parallel, in terms of artistic frustration, would be that of writers block.