Self..

..or the lack there of.  Okay, I admit it, I too am a tortured artist.  But don't you think we all have it in us to push beyond what we believe we are capable?  It is a completely aggravating and a completely rewarding process.  Perspective is priceless, but difficult to find.  I'm likely to get it wrong more often than I get it right, but I'm not going to stop.
Because the process of creativity is the reassurance that anything is possible.