A former McKinsey partner says if you can't make a dent in your to-do list, you're probably thinking about it all wrong

Every so often I'll try "time blocking": scheduling different tasks into my workday instead of writing them down and hoping they'll get done at some point.

In theory, this strategy is brilliant. And yet I rarely find it helpful.

What typically ends up happening is I organize my day beautifully: From 10 a.m. to noon I'll write a story; from noon to 2 p.m. I'll do research for another story; and so on.

And then it's 2 p.m. and I'm only halfway through that first story and I can't possibly understand what went wrong.

What went wrong, it turns out, is that I once again fell prey to the "planning fallacy." It's the tendency to underestimate how long it will take to complete a specific task.

Caroline Webb, a former McKinsey partner and the author of "How to Have a Good Day," shared her thoughts on the planning fallacy in an interview with psychologist Ron Friedman during the Peak Work Performance Summit.

Here's Webb:

"Rather than going back over every single instance of a similar task and looking at what the average is across that and then thinking about all the different variables for getting this task done, what you do instead is think of one shining example where everything went right, where you had no interruptions, you knew exactly what it was you needed to do, and you just got it done.

"Then that sticks in your mind as the idea of how long this task takes. That's the planning fallacy - that we use the shining example rather than the true average of our lives for how long this type of task has generally taken."

In other words, I remember that one time I banged out a story in an hour and assume that's how long writing a story always takes. I conveniently forget about all those times I started a story and then deleted the whole thing, so that it ultimately took three hours to finish.

The solution, Webb said, is twofold:

1. Estimate how long it will take you to finish one project. Now double that estimate - and add some buffer time.

2. Cut your to-do list in half.

Webb's reasoning is that, using these strategies, you'll accomplish most of what you set out to do. In turn, you'll feel happy, and motivated to do more the next day.

If, instead, you go home knowing you fell short of your goals, you'll be disappointed - and less motivated.

Here's Webb again:

"I'm not saying you don't have aspirations to achieve great things, and I'm not saying you may not have lofty goals. What I'm saying is all the evidence suggests you're more likely to stay motivated and enjoy the journey if you break the steps down into small, manageable pieces that make you feel clear you're making progress along the way."