Hey TW,
I once pursued a career in psychology (and have since abandoned it for real estate), but there was a principle that I found both impressive and disturbing that I think you might appreciate.
Basically, a series of research studies established that people suffering from depression were far more realistic in projecting what they were capable of doing in future endeavors, ranging from business projects to creative works, etc, while healthy individuals were wildly over-optimistic in their own abilities.
However, it was this very unfounded optimism that enabled the healthy individuals to occasionally succeed in doing something extraordinary and improbable. Because they were audacious enough to believe they were capable of superhuman feats, every once in a while they would pull off something deemed impossible or improbable by the researchers and the community at large.
So you see, while it's "rational" to be pragmatic in your assessment of what's possible, it is a case where reason actually inhibits your growth and progress. You're absolutely right about everyone else in the world being unreliable, and your business might be on shaky ground, and all of those other unpleasant thoughts - but those thoughts won't help you improve your business. Be unreasonable in your pursuits, and you'll achieve unreasonable results.
Hope that little social science pep talk helped!