Here's the thing about this saying, attributed to Lao Tzu (just below, in quotes), and others like it, which are very popular on the interwebs: It's (unintentionally) blaming people with anxiety or depression for their own anxiety or depression, and it's ignoring the importance of the body in experience.
"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."
Unless there's something I'm not getting, the gist is that if a person is depressed or anxious, their problem is they are not living in the present moment, that their thoughts are in the past or the future, and, therefore, it follows, their depression or anxiety would disappear if they would just bring their thoughts into the present.
So what's wrong with that?
A couple of things. One, both depression and anxiety are body states. It's no more likely we're going to change our depressed or anxious body state just by thinking differently about it than we are going to change our broken leg or pancreatitis just by thinking about it differently.
Once upon a time cognitive psychology told us thoughts come before bodily sensation, so adjusting our thoughts would fix everything. Then we got more sophisticated research tools, like fMRI machines, that could measure things like that. And guess what? No. Sensations come before thoughts. "Negative" thoughts are more often the result of trying to make sense of painful body sensations than they are the cause of body sensations -- like depression and anxiety!
Furthermore, depression and anxiety can be the result of energies left in the body after difficult or traumatic experiences, even ones we don't remember. The body remembers. In order to discharge that held energy from the body we have to work with it, not against it. We have to give the body the space to complete the cycle that was once thwarted or suppressed. Telling ourselves to get in the present moment won't do that.
If we tell someone they are depressed because they're living in the past, or anxious because they're living in the future, then they are going to feel like there's a flaw in how they're thinking, in how they're approaching life, and that's not helpful at all. Most people in these states, as a matter of fact, are very quick to blame themselves and conclude that they are just flawed, anyway.
In addition, people who are having post-trauma affects may find the present moment *the* most difficult place to be. That's where they got hurt, in some present moment somewhere. The process of coming back into the body, back into the present moment may then need to be approached very, very slowly and gently, and work done at every brief moment that presence becomes available to experiment with what feels safe, for a tolerance for presence to be built over time.
Plus, traumatic reexperiencing is, as far as the body is concerned, the event happening again, and because this phenomenon occurs in the autonomic nervous system it is outside of conscious control. It can be healed, but not by force of will or change of a thought or a practice unaccompanied by trauma-sensitive guidance.
Do thoughts have a role to play in our well-being? Of course. But at most thoughts are only half of the equation. Right now, we as a culture and the mental health field are out of balance, privileging thought and discounting the body, to our detriment. This saying and those like it are going around because they reinforce that bias and misunderstanding.
If there is an implication that someone will be cured of their pain or illness simply by taking "responsibility" or thinking about things differently, then there may be something more hurtful in it than people realize. Think about the rape survivor, the abuse victim, the survivor of a devastating natural disaster. How do they feel hearing that what happened to them is their responsibility? Or that they just need to think about it differently?
I am appealing to well-meaning people to consider more closely how some of these old (male, not that there's anything wrong with that, but very male) wisdoms (and certain New Age concepts) come off sounding, out of the context of their culture and larger teachings. Our Western interpretation may not always be what was meant.