Past is a memory, even 5 minutes ago is a memory. You can't change it. You can only recall it. Future is an assumption. It's completely uncertain and we don't know what to come next. But we have a present. It is the only moment we have. The only thing which we can control is the present. If we focus ourselves on the past we are just ruining our present by recalling memories and dwelling on them. If we worry about the future, we are thinking about something that "MIGHT" happens with a certain number to back it up. We live our lives like we are gonna live forever and never gonna die until we get this one call that reminds us that we have limited time. We don't regret the school we have gone to or the job we have taken. We regret that we focused on the useless things and wasted so much of our precious time. So please just live in this moment because it is the only thing we have.