The test was simple.
The participants, all non-exercisers with the goal of adding workouts to their weekly routine, were asked to write down how many hours they planned to spend in the gym over the next 2 weeks to begin creating their exercise habit.
So each participant entered in the amount of time he or she planned to spend in the gym. Then after the 2 weeks were up, they would record how much time they actually spent in the gym.
After careful calculation, the participants planned to spend about 20 hours working out over the next 2 weeks.
You may have gone through a similar planning process to these participants at some point. Thus, you probably won’t be shocked when I tell you that the average time actually spent in the gym wasn’t even half of that!
The participants in the study only spent an average of about 8 hours working out during the 2-week period.
But that’s not the most shocking finding in the study. After the two weeks were up, the researchers were curious to see if people would learn from their behavior.
So they asked the participants to start fresh and plan how much time they would spend in the gym over the subsequent 2 weeks. Surely, they thought, people would realize what was realistic this time around and plan accordingly.
Rather than learning from their past behavior, the participants planned to spend even more time in the gym! In their minds, the previous 2 weeks were an anomaly. And their “true selves” were going to show up and make it to the gym for 25 hours this time!
As you can probably guess, they didn't even come close. 
The Planning Fallacy
This experiment reveals what researchers call “The Planning Fallacy”. We are incredibly optimistic by nature. So when we set plans, we overestimate our own abilities.
Psychologist Roger Buehler discovered this in 1994 when he did an experiment with psychology students. 37 of his students were asked to estimate how long it would take to finish their Senior Theses. 
They were to make 3 predictions:
1. If everything went as well as it possibly could – which the students predicted would take them about 27.4 days.
2. If everything went as poorly as it possibly could – which they predicted would take about 48.6 days.
3. What they thought would actually happen – which they believed would take 33.9 days.
Then they were to measure their predictions against how long it actually took them.
The average actual completion time was 55.5 days – a whole week longer than if everything went as poorly as it possibly could!
Buehler theorized that the reason we do this is because we are all wishful thinkers. We want to believe that things will work out for the best, so even our worst planning scenarios aren’t that bad.
That is why despite a 40% divorce rate, newly weds believe they have a 0% chance of divorce; and despite a 30% chance of getting cancer, the average person only believes they have a 10% chance.
It's not that we don't believe that bad things like this can happen – we just believe that they will happen to other people, not us. 
How Your Optimism Can Hurt You
First, let me be clear, there are many proven benefits to being optimistic. Having a positive outlook will increase your generosity, your kindness, and your courage to take risks.
However, optimism that leads to the planning fallacy is a recipe for disaster.
What do you think the chances are that those participants were able to establish their exercise habit after 4 consecutive weeks of not hitting their planned goals?
Slim to none.
They probably felt like failures with every planned workout missed – leading them to give up hope.
The worst part is that you can clearly see that the 20+ hours of exercise were not necessary. Had they planned on even doing 5 hours of exercise over the two-week period it would have been a big improvement over nothing!
Yet, they didn't see it that way. They were too optimistic and they set themselves up for failure.
No matter what your goal, you will have a natural desire to overestimate your ability to accomplish it. You will look at the result you want, the free time you have, and you will genuinely believe that "this time" you will summon the willpower to do it.
HOW TO KEEP YOUR OPTIMISM IN CHECK
Unfortunately, you can't just keep your optimism in check by telling yourself to "be realistic".
In a similar exercise planning study, researchers found that participants who were told to "be realistic" actually planned to spend more time in the gym than those who weren't given any message! 
So what can you do to keep your optimism in check and actually be realistic?
The one proven way to prevent yourself from being too optimistic about what you'll be able to do in the future is to look to the past.
This is a strategy called "reference class forecasting", which is simply taking the results of the past and using them to plan for the future.
If the participants in the first study had used the first 2 weeks - where they exercised for 8 hours - to plan for the next 2 weeks by upping it to 9 or 10 hours, they would have used this method. And they would have had a much better shot at creating the habit!
This works because it helps you avoid the trap of thinking that the future is going to be “special”.
We all have an idea that in the future we will have:
The list goes on. But your future self is not a superhero. In fact, they're probably going to be pretty much the same as you are today. With a finite amount of time, a finite amount of money, and a finite amount of willpower.
When you plan for the future, you need to understand that it will be much more like the past than your natural optimistic projections. 
Once you take that moment to be self-aware, then you will be in the right mindset to create a simple, realistic plan to execute on your goals - without killing yourself in the process.
You must understand that you are a member of an incredibly optimistic species.
This optimism has helped us dream big dreams and accomplish incredible things. But sometimes your natural optimism about the future can leave you feeling like a failure if you don’t live up to your own unrealistic expectations.
To avoid this, you need to look to past results as the best predictor of the future. Then, if you end up doing better than you have in the past, it will give you a great sense of pride as you see real progress in your life.
So don't set yourself up for failure. Look to the past, set your goals, then crush them.