There are two common mistakes here:
Frequently reconsidering decisions that ought to be left alone. Once you enroll in college, it is both painful and a waste to spend the first five minutes of every morning wondering if you should drop out or not. Once you've established a marketing plan, it doesn't pay to reevaluate it every time your shop is empty. And once you've committed to a partnership, it's silly to reconsider that choice every time you have a disagreement.
In addition to wasting time, the frequent reconsideration sabotages the effort your subconscious is trying to make in finding ways to make the current plan work. Spending that creative energy wondering about the plan merely subtracts from the passion you could put into making it succeed.
On the other hand, particularly in organizations, failure to reconsider long-held decisions is just as wasteful. Should you really be in that business? Should this person still be working here? Is that really the best policy?
Jay Levinson used to say that you should keep your ad campaign even after your best customers, your wife and your partner get bored with it. Change it when the accountant says it's time. And Zig Ziglar likes to talk about the pilot on his way from New York to Dallas. Wind blows the plane off course after a few minutes. The right thing to do is adjust the course and head on. The wrong thing to do is head back to New York and start over (or to reconsider flying to Dallas at all).