The present moment is really the only thing that we need to live in. What an easy statement to write and a hard statement to live. We are by no means going to be able to practice this princeable with perfection.
We worry, we're human. We think about the past and how we wish we could change some things about it. Well, that is never going to be possible. I heard someone say that worrying about the past is one of the biggest wastes of time in the world. There is nothing that we can do to change our past. Our pasts have made us what we are today. If we are sober today, and living a life beyond our wildest dreams, it would not be possible if we didn't have something to compare it to like the past.
Worrying about the future is probably the second biggest waste of time. We are, by nature, not the best projectors of the future. When we are thinking and worrying about the future, we usually make it out to be much worse than it is going to end up anyway. When we worry about the future it is almost like we have taken our will back, and we are again managing the whole show.
The present moment is the easiest to live in.