Join the club, my friend. Being a writer is a lot like being a baseball player: even the best ones fail most of the time they step up to the plate. I think most writers would agree that if the act of writing isn't challenging technically and intellectually — and often emotionally — you're either not working hard enough or you're on mood-elevating drugs.
If writing were easy, what reward would there be in doing it? How could we judge when it's done well and when it is not? Try not to be dismayed or deterred by how difficult it is to distill your thoughts and get your words down right. Instead, whenever you sit down to write, think of it as an opportunity to suffer productively (if not lucratively). And summon as your muses those "friendly sparrows" who, when Peter Rabbit got stuck in the gooseberry net, "implored him to exert himself."