Let me make a prediction about you. You are always thinking about something. When you’re eating, walking, exercising and partying, your mind is constantly racing as you replay the past and plan for the future. Tell me I’m wrong. Seriously, I would love to know about people with a naturally calm mind. I only recently realized that this appears to be the default case, with the exception of strongly emotional moments, for the vast majority of people. And being in a modern stimulative environment certainly does not help.
The problem is that we are spending our lives focusing on things we have no control over. Learning/re-experiencing from the past and planning for the future are both very important tasks, but they are certainly no more important than actually living and experiencing life; being in the present. While we spend our time imagining past and future scenarios, our lives are going by before our eyes.
Being present is about focusing on your immediate existence and clearing your mind of anything but immediate stimulus. It is easiest to do when you are doing one thing at a time; try eating and just experiencing the food, or running and listening to yourself breath. Just like any skill, being present takes practice and you can improve over time. You can achieve a sensation similar to the effects of THC where your mind is completely focused on a simple, routine and surprisingly novel stimulus. Give it a try, experiencing the present is a very relaxing state of mind.