The Dip by Seth Godin is a short and solid book about the decisions we make to either stick with things or quit them. He makes the point that single-minded determination and a never say die attitude not always a good thing, it often is better to move on from something and focus energies elsewhere.
The idea of the dip as Godin describes it is the hard period of something, the long slog between starting and mastery. Almost everything has a dip. It’s supposed to be there and is in effect a barrier to entry that creates value. Also, Godin points out that many professions have superstars and then a bunch of also-rans. Second or third place isn't a terribly successful place to be. The goal really should be to be the best at something, even if it a small area, not to muddle your way to an ok level of competence. Pushing through the dip is a good thing, what you don't want to do is quit in the middle of the dip, when it's hard, but still worth it to succeed.
It's noted that along with dips, there's also cul-de-sacs, spots where no matter how hard you try, things won't get better. Those are the dead ends that should be abandoned. Godin also points out that coping is a lousy alternative to quitting. All coping does is waste your time and misdirect your energy. If the best you can do is cope, you're better off walking away. Highlighted is that winners quit fast and often, and then beat the right dip for the right reasons. Quitting the things you don't care much about, are mediocre at, or aren’t going anywhere (a cul-de-sac) frees you up to push through the dips on the things that do matter. When you are ready to quit something, go for broke, be willing to ask for what you want, and willing to walk away. Going into new situations lets you reinvent yourself; you've left behind those who have branded or pigeonholed you.
The right idea isn't "never quit," the right idea is "never quit something worthwhile just because it's hard at that moment." Don't quit your strategies, quit your tactics, and remember that a particular job is tactic, not strategy. Getting through the dip is never quitting the big idea. "Quit the wrong stuff. Stick with the right stuff. Have the guts to do one or the other."