I strongly believe that there are times you should call it quits on a business. Not everything works. And — even after trying incredibly hard, and for a long period of time — failure is sometimes the best option. An entrepreneur shouldn’t view their entrepreneur arc as being linked to a single company, and having a lifetime perspective around entrepreneurship helps put the notion of failure into perspective.
Entrepreneurship is a career (and a safe one at that). But that doesn't mean that every startup you start has to succeed for you to be successful.
Quite the contrary. Both of my first two startups failed. My third company is now successful, highly profitable with 11 employees and growing, and so I am generally considered a successful entrepreneur now.
The only constant about declaring that your startup has failed seems to be that everyone wishes they'd done it six months earlier. It's easy to know when you're succeeding, it's much harder to know when to quit and try something different.