In The Dip, Seth Godin advocates greatness. He helps you identify when the endeavor you've undertaken is in a cul-de-sac (read: headed nowhere ... so quit before you've wasted any more precious time) or whether it's just facing what he refers to as "the dip." If your product or business is in a "dip" you need to work on some things that aren't working, but essentially the "dip" can be overcome. He argues that most people either 1) quit too soon (just before they've overcome the dip) or 2) don't quit often enough (because if you spend too much time on a "cul-de-sac" project, you've wasted time where you could have actually done something remarkable). The bottom line of the book is that you shouldn't settle for being anything less than the best and remarkable, and that WILL take incredible perseverance, but you can't be the best at everything, so choose wisely.
It really is a great read, and at only 80 pages, it won't take you very long to get through, so you honestly don't have anything to lose.
Since I'm sure I have undersold this book, as I'm not quite as brilliant as Mr. Godin, I will leave you with a quote from the book:
"All our successes are the same. All our failures, too.
We succeed when we do something remarkable.
We fail when we give up too soon.
We succeed when we are the best in the word at what we do.
We fail when we get distracted by tasks we don't have the guts to quit."
Also check out his blog that specifically relates to issues addressed in The Dip: http://sethgodin.typepad.com/the_dip/