I have contrarian views - most of the time, but usually agree with Guy on almost everything. Until now. In my view, the essential question (when do you quit?) is a very important one. It is hard to distinguish the difference between focus & discipline, and being smart enough to adapt and change or innovate.
The difference between success and failure are tallied in the decisions you have made. Only with hindsight do you know if a decision was right or wrong - and that is why so many people believe in 'luck'. Sometimes 'sticking to a decision' gets the right outcome - much like a broken clock is 100% right at least twice a day. Sometimes you have to 'desert a sinking ship' or 'stop throwing good money after bad'.
In this case, the question has been answered by a statement that begets even more questions: When you have suffered enough? When is the pain the greatest and how do you know that pain won't get more (or less) soonish? Seth is fishing at the shallow end of the 'achievement' pool... and we are none the wiser. I believe there is an answer, but it ain't Seth's little aphorisms.