I really hate this cliche when applied to startups. Here's how I see it:
The night is darkest just before the dawn...or before it somehow gets even darker.
We read a lot of "I stuck it out 1 more day and then we got rich" stories but it's the same survivor bias that we see in all the startup stories. I would wager that for every 5 companies that "just don't give up", 4 of them eventually give up. But, the 4 that give up don't write self-congratulatory blog posts and give interviews about their failure. So, we only hear about the 1, never about the 4.