The nineteenth-century Scottish fantasy writer, George MacDonald, lived in intense poverty, writing fairy tales in order to eke out a living for his family. Yet he had a peaceful mind. C.S. Lewis said of him: “His peace of mind came not from building on the future but from resting in what he called 'the Holy Present.'"
The Holy Present is a mode of living in which you don't think about the past or future. You think about the job at hand or an old tree you see in a park. You think about the game you're playing with your children, or the conversation you're having with your friend, or the consultation you're having with your customer or client. Or you might just think about God. You don't think about the time being "lost."
You should strive to live in the Holy Present because, quite simply, it's how we're meant to live. We have no power over the past and precious little control over the future. Here's how C.S. Lewis described it in The Screwtape Letters: God wants men to attend chiefly to two things: “to eternity itself, and to that point of time which they call the Present. For the Present is the point at which time touches eternity. Of the present moment, and of it only, humans have an experience which (God) has of reality as a whole; in it alone freedom and actuality are offered them.”
If you learn to live in the Holy Present, all sorts of problems fade away. Anxiety vanishes. The idea of not having time becomes foreign. Concern about money fades away. Worry about future events–terrorist attacks, stock market crashes, illnesses–crumbles. Resentment or sorrow over past events never surface.