re: Why has mental health declined so much over the years?

The first part of your question is that people are way too connected and invested in the image of other people. Social media allows for the careful crafting of a public image by pretty much everyone. People can't help but judge their lives against the perceived lives if others. Couple that with the fact that most people simply cannot unplug, ignore it, and live their own life. That leads to depression and anxiety at levels we have never seen.

To the second part, as to why mental health issue are so prevalent, we have neglected treatment and management. We have gutted public mental healthcare in this country, shifting it to the private sector that is out of reach for far too many people. Treatment just isn't accessible. What is left of the public care is swamped, and it takes way too long to get issues addressed.

The result of that is the burden of the mentally ill has now fallen, by and large, on local law enforcement. They are not trained, nor equipped to handle it.

Of course, as evidenced by going from general depression and anxiety in my first paragraph to more serious matters in the last, there is a significant spectrum of mental health having to be dealt with.