In the privacy of your thoughts,
Think of something in the future, that you worry a lot about.
Think of something in the past that you regret.
Think of something in the future you wish to happen.
Think of a time in your past, you wish you could return to.
Now try to focus on the present moment, as it is in your life right now.
Notice how wishful and regretful thoughts try to distract you from appreciating the present moment.
The actual experience of life can be completely missed by dwelling on thoughts about the past and future.