Wild creatures live exclusively in the present. They do not anticipate the future nor dwell on the past. Since their lives are governed by instinct and learned behavior, they have no worries and no regrets.
Humans, on the other hand, are often consumed by their past and their future. Young persons can't wait to be older and the elderly mourn the loss of their youth. Those of us in the middle mull over past decisions and worry about future challenges; the past usually seems better than it was and the prospect of aging and dependence weigh heavy on our minds.
In my experience, it is our inability to live in the present that usually leads to unhappiness. The grass was and might be greener. But, while it's good to learn from past mistakes and to plan for the future, the past cannot be changed and the future may end tomorrow. It is best to enjoy your health, your passions, your family and your friends while you can. A commitment to live in the present is a gift to yourself.