The biggest waste of time is thinking about yesterday and tomorrow. We can hardly do what needs to be done today, let alone wasting so much time about the past and the future.
I have to be really specific here. You can’t gain wisdom unless you learn from the past. All experience comes from the past, so I’m not saying to forget the past. I’m saying quit wasting time thinking about the good old days. Take from it what you need and go forward.
The same holds true for tomorrow. It’s okay to have plans for the future, the problem is that too many people keep making plans and dreaming about their future but they never do anything about it. Work towards your plan and goals but do what you have to do today. It’s the only moment that exists.