Is Believing Enough to Make Something Happen?
Introduction
Is believing something will happen enough to make it happen? To what degree is the power of positive thinking a myth, and how much of it is real?
When does believing it won't happen enough to prevent something from occurring?

How Much Wishing Will or Won't Make It So
Taken to its extreme, sitting under a tree wishing for a pot of gold will not make you rich.
Let's look at a real case scenario. Wandering through your life believing things will happen without taking any effort to make it reality will not lead to it happening. Even if you believe you'll win the lottery, you must still buy a ticket. And in that scenario, wishing, praying, believing and hoping with all your heart will not actually increase your odds of winning.
Believing will not make something happen without effort on your part, and the hard work is what puts off many people. Instead, they retreat to the power of positive thinking, they read "The Secret" and assume positive affirmations are enough to make their dreams come true. And wishing and believing without effort will fail because you must actually invest effort to make things happen.
Why Do We Think Wishing Will Make It So?
We tell stories about the improbable, like the person who challenged God to hit him with lightening and was. No one tells the stories of people who made such dares and nothing happens.
People tell the stories of the person who won the lottery in dire circumstances - such stories even make the news. They credit believing, praying, hoping. The stories of those who won the lottery and lost it come up from time to time but not as often as the human interest stories that give us a false impression that the unlikely is likely to happen to anyone. In these cases, our society encourages humility so you see praise to God, good luck, advice, winning systems instead of someone saying, "I'm better than you." This leaves us with many heart warming stories of "just believe and your dreams will come true"!
This view is compounded by the books that say believing is enough to make it true. After all, few people want to buy a book that says you have to work long and hard to achieve your dreams. Making a plan and following it, to quote Dave Ramsey, is the definition of maturity. But the fact that get rich quick books and seminars saying positive thinking alone will make you wealthy testifies to the fact many want the easy route out - the wishing really hard, believing with all their heart, the daily affirmations and little else will make their dreams reality.
How Negative Thinking Will Hurt You
The opposite of "will believing make it happen?" is "will saying it can't happen guarantee it won't?" If you hold the belief that it will not happen and never put in the work to make it happen, you've failed and the belief manifested as reality.
What if you do the work but believe it won't work? If your work is shoddy and your attitude poor, you're likely to fail. Thinking that the job is horrible and you'll never get promoted or see a raise hurts your performance, and that in turn guarantees you won't see a raise or promotion. If you think the project will come out badly, your half-hearted effort nearly guarantees that poor outcome.
In a personal example, I'm a published author, dozens of stories in anthologies, several books of my own. If I thought it wasn't possible, I wouldn't have sent the first several hundred submissions before the first acceptance. If I didn't think it was possible, I either wouldn't have tried to submit the stories at all or would have sent it to a few publications before giving up.

The Balancing Act Between Belief and Action
If you believe it isn't possible, you won't try and automatically fail. If you believe it won't work, you go through the motions until you give up - and fail.
What if you try to act without a plan? You might succeed, and then lose it all. Think of the people who go on a strict diet, lose 30 pounds, and upon success, resume old habits and gain it all back. Or the people who receive an inheritance or bonus at work, revel in it, and spend it. Now they are poor again and haven't made progress on paying off their debt in most cases.
The solution is to make a plan you can follow and then follow it, believing it is possible to achieve. Creating a budget and following it, paying off a chunk of debt each month isn't sexy -but it works per gurus like Dave Ramsey and Suze Orman. Setting up a workout routine and changing your day to day habits to get more activity is isn't as exciting as saying "I'm thin and beautiful" ten times each morning or as dramatic as drinking awful smoothies each morning. Yet the long term plan of paying off debt and then saving toward goals is much more likely to work if you see the end in mind and work for months toward it. The goal of reaching a healthy weight is more likely if you eat healthier and work out more each day for years, as are the odds of staying there.
And believing something is possible is what will keep you on track when there is a setback. If you are only going through the motions, the first set back or failure leads to giving up - and then you've failed.