The frustration over hack writers.
Everyone is a writer these days. Mention that you write and you'll probably get a stream of people coming out of the woodwork, telling you that they write, too. What have they written? Oh, nothing, nothing since college at least. But, man, the ideas they have in their head! Just wait until they get a moment to write them down, because the world is going to be flipped upside down!
It's frustrating because it trivializes what you work so hard to achieve. No, that's cool; interrupt what I'm saying to tell everyone you write as well. I'll just stand over here and find a wall to kick.
The scary fear that you might be a hack writer, too.
Who is to say that you are not that hack writer as well, swearing up and down that your shitty writing is going to change the world (or at least the lives of a good number of people), when, in reality, your drivel is sophomoric and dull? And getting your work published is no comfort; just look at Stephanie Meyer, EL James, Cassandra Clare. They could fill swimming pools with the money they've made from their books, and all three are considered blatant hacks by everyone from literary critics to that dude next door.
The ritualistic mutilation of your ideas as they go from your head and into print.
Dammit, this idea was perfect in my head, outside of any context or language or realm of believability. Why is it shifting and changing as I try to write it down? Why can't I get my character to sound realistic? Why does this suddenly suck? Am I nothing but a person who has good ideas in his/her head, but can't ever get them down on paper (see frustration #2).
The ritualistic slaughter of your darlings.
That hilarious bit of dialogue between two characters? It bogs down the story; axe it. That pivotal scene that sets all the other events in motion? Weak; rewrite it. That incredible character that you love oh so much? He is useless; delete him. That downright poetic description of the room? Too flowery; kick it to the curb.
The universal rejection by the powers that be.
Agents love your work, but "can't take you on as a client right now." Literary magazines enjoyed your story, but "feel it isn't a right fit for their publications." Website producers love how you write, but are going with someone who "has a different style and voice." You can cast the widest net and still get the same responses over, and over, and over, and over, and over again. You just want to shake everyone by the shoulders and scream, "GIVE MY WORK A CHANCE, DAMMIT!" Which you then rescind because you are constantly experiencing frustration #2.
The inevitable comparison to successful writers.
It's one thing to be 21 and looking at all those super-successful novelists with their millions. It's another to be 27 and realizing that some of those novelists already had their first book out by now. You turn on the TV and see the media saturated with people who are at least two years younger than you. It leaves you slumped against your seat, palms in the air, questioning your life and your choices (again going back to frustration #2).