I think he's talking about passion/inspiration and having to work for it...some days it don't come easy, (inspiration doesn't come easy)
Some days it don't come hard (sometimes the passion/inspiration is there, but not enough of it- it doesn't come hard and fast, so to speak -
don't even say it pudding-
)
sometimes it don't come at all, and these are the days that never end (well yah, when there is no passion and inspiration it's beyond frusterating...it's heartbreaking)
That analogy fits it with the next part as well, so that's how I interpret it...
so, basically, I agree with Chris- other than my interpretation of what it means when it 'doesn't come hard'