Oh yes, I read that one too. In fact, it inspired me to write a newsletter (Thursday!).
I wonder if you ever thought that the very question "how do we know when to quit a project" is kind of unproductive?
In therapy there is sometimes called a protective behavior. When you don't trust yourself and you're guarding yourself against potential failure and icky feelings connected with it, you feel that by thinking of a worst case scenario you will actually be ready for it (and protect yourself against discomfort).
Say if I were getting married and I thought, "I wonder when to get a divorce" that might actually "help" me subconsciously prepare myself for it, look for evidence that "it's that time" and maybe even create more evidence as a result.
What if when we changed the way we think going into a project? What if we didn't consider every failure as "a sign showing us it's time to quit" or "not meant to be" but rather a stepping stone to success, a way to improve, a way to grow ourselves through it?