As I've gotten older this maxim means more and more to me. It's not just about getting more done or being more productive it's often about futility of life vs. the doable of life.
I've heard the serenity prayer for ever. In fact I've heard it so much I don't really hear it anymore. By now when I hear it all I hear is "Don't waste time"
If God is going to grant me the wisdom to accept the things I can't change; the courage to change the things I can and the wisdom to know the difference I need to get to work.
Don't waste time worrying about how others think about you. Their definition of you is an external fragment of themselves; something over which you have no control: don't waste time worrying about that or attempting to change their opinion. It won't work. This is part of what lives in the futile category.
By the same token don't allow that externalized fragment to define your internal world; the world over which you have complete control.
You are the only person who has the opportunity and the duty to take full control of and full responsibility for you. Never put this off and never give the job to anyone else.
If I can do something about it I need to get to it. Now. I may be too old. That doesn't matter. I should have done it in my 20s. That doesn't matter. It might not work (probably won't) that doesn't matter. Can I do something about it? Yes. Does it need doing? Yes. Do it. Don't. Waste. Time. There's something in process that is important-more important than outcome. Outcome is in the future. The future is speculative. Don't trade speculative for now; don't waste time.
The nature of life boils down to what you can control vs. what you can't. Those items that are futile that have been ripping your insides out? Let those go. Don't waste time.