Pursuit of Passionate Purpose
Any entrepreneurial venture is difficult at best. What gets you through all of the tough times is having a singular vision of what you're trying to achieve and then pursuing it with great passion.
Theresa M. Szczurek, one of the founders of Radish Communications, has written a book entitled Pursuit of Passionate Purpose: Success Strategies for a Rewarding Personal and Business Life that is billed as "a practical guide for producing extraordinary results and reaping the real rewards of life by caring passionately about what we desire and then pursuing it with true determination."
I attended a talk by the author at the Boulder Book Store out in Boulder, Colorado, and yes, the author really does come across as being quite passionate about this stuff, but there is also a reasonable amount of down-to-earthness to it (she started out as an engineer). She offers a lot of specific and practical strategies and many illuminating anecdotes contributed by a number of successful entrepreneurs that she interviewed.
In any case, the real point is that you find something that you're passionate about and pursue it passionately, and once you've achieved your goal, your vision, and have satisfied your passion, move on and find a new vision to be passionate about.