Let's take a step back for a minute to understand a little bit about mental illnesses. First, set aside whatever beliefs you have about nature vs. nurture, drugs vs. therapy, 'real' vs. 'imagined.' Because it doesn't matter.* It doesn't matter whether depression is caused by a chemical imbalance or the death of a loved one. It doesn't matter whether you are anxious because of a genetic predisposition or because of a recent stressful event.
* Depression, anxiety, and other disorders can be caused by other illnesses such as over-active or under-active thyroid, diabetes, and others or can be a side effect of medications. That matters very much as they are side effects of another illness that needs to be treated.
If you walk into your kitchen to find a koala on the counter, it does not matter how it got there or how you think it got there. You can call it it parrot instead. You can think that opening the window and leaving for a few hours is the best way to get rid of it. None of that matters. The fact remains that there is a koala in your kitchen. Depression, anxiety, bipolar disorder, ADHD, obsessive-compulsive disorder, and many others exist regardless of how you want to define them.
A Better Example Perhaps
Last night I dreampt that I had a rear wiper on my car. When I get in my car this morning, I knew that I did not have a rear wiper. I knew this because it didn't have one yesterday or any of the days before. I also knew this because I walked behind the car to get to the driver side door. Once I started the car, I looked at the lever that controls the wipers just to be sure that I hadn't somehow missed the switch for the rear wiper. 20 seconds later, I tried to turn the rear wiper on.
I think it's safe to say we have all had a similar experience. A dream that was so real, when we wake up we're not entirely sure it wasn't real. Even if we know it was a dream and it couldn't have been real, we often check anyway.
Or This One
Have you ever been watching a movie and noticed that you were tensing up, holding your breath, that your heart was racing, jumping when something startling happens on screen? It's not real, but your body reacts as though it is. When you realize that you might relax and start breathing normally again, but your pulse doesn't immediately drop back to its normal rate. That's because you can consciously control your muscles and breathing. You can't control the release of adrenaline or prevent various organs from reacting to the surge of adrenaline. The way you treat/cope with/calm down from being startled in a movie is the same way that you would treat/cope with/calm down from being started by something in real life.
Putting aside the world of health insurance, medical research, and policy, consider that perhaps what you call it and what causes it does not matter. Perhaps when it comes to understanding friends, family, or yourself, what matters is simply that our bodies are reacting to a perceived situation and simply acknowledging that is not going to instantly get us back to a normal state of being.
But My Koala
Getting back to that koala. Or not.