Have you ever wished you could go back and do things differently in your life? Regardless of your age, and whether in your career or personal life, there are probably choices you made, or attitudes you held, that you wish you could revise.

When O magazine asked actor and activist Michael J. Fox (along with 16 other celebrities) to write a letter to his younger self, they couldn't have predicted the poetry that would emerge. What he would say about those choices and those attitudes. 

In 1999, Michael J. Fox announced publicly that he had Parkinson's disease. In 2000, he left his role on TV show Spin City to found the Michael J. Fox Foundation for Parkinson's Research. In the intervening years, he has done much to raise awareness of -- and funding for -- the illness.

The poignancy of his situation lends that much more weight to his letter to his younger self. One phrase in particular jumps out in terms of words to live by.

"When the time comes to chase your dreams, and it will, they may seem elusive," he said in the letter. "[B]ut know you won't catch them all at once. Just one challenge at a time."

Just one challenge at a time.

We live a complex and interdependent world. We balance our concerns over our own personal finances with concerns about climate change. Does it really matter what we shift around in our portfolios if the polar ice caps are melting?

Even smaller concerns seem to loom large when you're trying to figure everything out at once. Do you stay in a job that stresses you out until you have another option lined up, or do you quit on faith? Do you get a part-time job while you're founding your startup for the financial stability, or will that distract you from your venture?

Fox's advice has, if anything, only grown in its relevance. Because you simply can't move forward when you're trying to do everything at the same time. You've got to pick one or two things to focus on, do those, and then trust you'll address the other things when they come up.

This isn't to say you should procrastinate, but it is to say that being discerning and focusing on bringing one or two things to completion is far superior to maintaining an endless to-do list that you feel overwhelmed by every time you look at it.

In other words, whatever you're doing, it's enough. You don't have to move mountains. You just have to make steady progress, day by day. Just one challenge at a time.

Here's the full letter:

Dear Michael,

When the time comes to chase your dreams, and it will, they may seem elusive; but know you won't catch them all at once. Just one challenge at a time.

When success comes, and it will, don't gobble it up--savor and share it, and it will last.

When love comes, and it will, don't bury it in expectation and projection--be prepared to fall in love all over again every day.

When the unexpected and inconceivable intrudes on life, and it will, deal with life's actual events--don't obsess about perceived eventualities.

Relax--enjoy the ride.