If you are depressed you are living in the past.
If you are anxious you are living in the future.
If you are at peace you are living in the present.
As someone who has lately been bouncing back and forth between these states of mind, I can appreciate the essential wisdom of the quote. Most of my feelings of depression lately have been spurred on by my regrets about things I wish I had done differently in my life, and so in that regard, they are an instance of dwelling in the past. Most of my anxiety stems from my concerns about where our country is headed under its current leadership (or lack thereof), and my feelings of uncertainty or even paralysis as to what is or should be the best path for me to take from this point forward. The more I think about it, the more overwhelming the many different options start to become.
Some of us are broken. We were given both envy and high principles. We can’t even contemplate bringing others down to level things, but instead we work madly to increase our status. (No, it’s not how I think about it, but it’s probably what’s going on in the back of the monkey brain.) Most of humanity however is functional. Give them enough to eat, and a place to live, and no matter how unvaried the diet and how small/terrible the place, most people will stay put.