On those days, reading most writing advice feels like a kick in the gut. Knowing that all my favorite authors kept on writing when they faced difficulties, producing thousands of words despite all adversity, is just a reminder of how weak and pathetic I seem. Thinking about that is sure incentive to give up.
Luckily, the fire to write is like any other fire: I've fed mine so much fuel over the years, that even smothered, the coals burn hot. Even when I feel like crap, that fire is still smoldering, churning over ideas and dreams. To get through these times, the best thing to do is to stop trying to shift the rocks and simply feed the fire. It'll burn through any obstacle, given enough attention and kindness.
Because I cope poorly with adversity, I might never be a successful writer. (Because of that, I might never be a successful anything, really--isn't that what the school counselor told me?) But I think about some of the great authors who've lived, people like Louisa May Alcott. Sure, she was tough and smart and funny--but when things went wrong, there were plenty of times when she climbed into bed and didn't get up for days at a time. She still managed to scrape by as a pulp fiction writer (until her amazing success with Little Women). She never gave up after her many set-backs.
So I think the single most important piece of writing advice you'll ever get--more important even than "Butt In Chair," which is 90% of everything a writer needs to know--is that you are the only person who can keep your dream alive, and you must do everything in your power to feed it and nurture it. You'll never be a writer if you let the dream die.