This has always been my issue with psychology. It is inherently unscientific. Given the preponderance of reported mental conditions, it's also insufficient.
My experience is that the patient is asked a series of incredibly open ended questions. I was struck with the sheer variety of ways that I could respond. And depending on my response, I would receive a diagnosis.
The following were my diagnoses from 3 different psychiatrists: Attention Deficit, Seasonal Affective Disorder, Bipolar disorder.
There was absolutely no consensus, and it's no wonder. Years later a doctor made the connection between chronic pain (Crohn's disease) and my lack of energy.
I had been consuming anti-psychotics purely because of how I answered "How are you feeling today?" and other similar open ended questions in the wrong way.
At that point I worked to improve my chronic pain condition, which was of course the actual issue.
Neuroscience is the way forward to understanding mental disorders. In the meantime, great care should be taken around psychology. The effect of an uninformed approach can be devastating.