We all have it. (And if you don't maybe you should hang up your pencil and go experience some LIFE instead?)
It comes on many levels, but I'm only going into the one that I'm being currently reminded of ... I'm nearing the end of the current rewrite, and shortly I will have to put pen back to paper to embark upon/complete a new story.
You've finally written something you really like (and rewritten it to death. ARGH!*). But now you have to come up with a new story. It can't possibly be as good as the last one. You can't write, heck, you've forgotten how to spell the word "and" - you know it's short and it's got three letters, and there's an 'n' in it somewhere; but can you remember how to spell it? No.** For that matter "it" is a little perplexing too.***
There is no way you can come up with something that is anywhere near as good. You've read the first story back, and you don't have a clue how you came up with it. There are words in it that you didn't even know you knew. It's far too good to have possibly been written by you. But you remember doing it. You remember the joyous little victories of coming up with the perfect word, that sentence structure that was so crucial to making the story flow perfectly, the weird dreams that you woke up with in the morning that allowed you to crack this or that character / plot problem, which you then feverishly wrote ASAP before breakfast. AND THERE IS NO WAY THAT YOU WILL EVER BE ABLE TO DO IT AGAIN!
It's so depressing, you should give up now!
Except you can't. Because then you'd go properly insane, rather than just be the weirdo nutter that you are now. And you know how twisted your innermost thoughts are. Releasing those upon the world in any form other than fiction ... well a gibbering drooling wreck in the corner of a padded cell is possibly a preferable option.
So you have no choice.
You have to keep writing.
It's who you are.
[One day I have to read Stephen King's book on the subject. Yeah, I know, I should have bought it when it first came out ... but it seemed that little bit too close to home for comfort. And I'm still preparing myself for that journey. - - It's probably no where near as bad as I imagine. But, what if it's worse? ARGH!]
So you write, because that's who you are, but you don't know how, and that not knowing is terror! It calls into question your very reason for living, for being. And, what if one day, today? you find you can't plumb that well? Except you force yourself to sit down. Bum on seat. And have at it! "Once more into the breach..." and all of that. You have to.
And that's just one form (among many) of The Agony Of Writing.
*I suspect rewrite will be Part II of The Agony Of Writing)
**The Ancient Greeks had a part of grammar called the optative. I wish we had it in the English language. I'd use it all the time. ... It allows you to write a question and an exclamation at the same time without having to faff about punctuation. I suspect it would also allow you to write a rhetorical question without any of the confusion of misconstrued punctuation, as well. Okay, I suck at grammar - and at Ancient Greek - but this is such a useful little writing knick-knack, we really should have adopted it. Like 'Zero' is such a useful little knick-knack for maths.
***This really happened to me, shortly after I'd completed my English Language 'O' Level, during a class assignment. I froze, stared at the page, eventually thought "B*gger it!* left a three letter space and wrote the rest of the paragraph figuring I'd remember how sooner or later. Sure enough, three paragraphs later (or it may have been one?) the 'spelling' inflicted itself upon my maladjusted brain. You try writing three paragraphs without using the word "and"....