When to quit and when to stick?

Out of all the books I am currently reading, I found two on my desk at the same time, that were in striking contrast to each other.

Firstly, Seth Godin's, 'The Dip'.  Less than 100 pages, and I read it on a morning off, in one sitting over two cups of coffee.  One simple idea, that we quit when we should't and don't quit when we should and how to know the difference.
A book so simple, that is seemed bizarre no-one else had written it, or for me to have realised the obvious contained within it.  But then all great and simple books seem that way.
It left me thinking about some of the dips I am in currently and want to persist through:
1.  Exercise and Weight: After 3 years, losing 50lbs in weight, and training regularly I'm much healthier, but stuck in a 'set point' that I can't seem to get through.  I'm in a dip through which I must continue.
2.  PhD: Three years into a 6 year track, overwhelmed with workload, a child with special needs, teenagers who I want to be around more for, and I am in a huge dip.  Wondering if this will ever end.  But Mr Godin helped me see that this is a normal dip, and reasons for keeping on, keeping on.  
3.  Church:  Church planting has so many lows and highs, and plenty of dips.  The tidal wave of consumer life that demands attention for everything like a black hole, seems to make the idea of building a community who do life together almost impossible.  I'm in a dip (more than the church), and realising that prayer, and faith, and persistence are what's needed.
Thank you Mr Godin.
Contrast that with another book I have just read, 'A Mad, Bad, and Dangerous People?: England 1783-1846', by Boyd Hilton.  760 pages, a life's work that is so dense and profoundly perspicacious that is sits mockingly next to The Dip.  Impossible to write, not a times best seller, and probably read by a handful of people.  Yet a book that helped me see my life in the context of the history of my own country and my faith.