You should feel that you have permission to write the worst junk in the world and it would be okay.
--Natalie Goldberg, Writing Down the Bones
I don't always agree with Natalie Goldberg's approach to writing, but I wholeheartedly agree with the philosophy of giving yourself permission to suck.
Writing is hard. It's humiliating. And, for me at least, my best writing manifests at such random times that the conditions under which I write well seem totally arbitrary.
That's why you must make a habit of writing--every day. You have to show up, to borrow a phrase. And suck, if that's what ends up happening. Then, you have to show up tomorrow and write again. And the next day. And the next.
At some point in there, you will write something exceptional. Keep showing up and it will happen.