https://jonathanhaidt.substack.com/p/mental-health-liberal-girls
It lays out the facts on the rise in mental health issues, heavily concentrated among liberal/progressive girls, since 2012; addresses some common (but honestly silly) explanations; and offers a hypothesis that is highly compelling.
The short version is that academia and media are performing a form of "reverse cognitive behavioral therapy", encouraging kids to wallow in misery and depression, focus on the worst things and most contentious things that are happening in the world (many of which are even imaginary), and destabilizing their ability to cope effectively while the rise of smartphones and Instagram in particular dragged teen girls especially into patterns of narcissistic isolation.
Meanwhile... Social media, in general, has the effect of amplifying strife, which only makes the world seem worse.
A coupling of bad philosophy + technological changes creates a massive problem, and while Haidt probably wouldn't put it this way, it's created a large percentage of rather unhinged kids - mainly female, and mainly in the progressive camp, which may explain the explosion of cancel culture and angry activism on the left.
You have one group that offers at its philosophical core a sense of nihilism and uncertainty while actively promoting the idea that everyone has their own "truth" and that how people feel about a situation is synonymous with the reality of that situation. This group has also told people for the last couple of decades that they live in a horrible world filled with evil and malicious people who want to oppress them and within that sub-culture, being a victim confers added social status.
There's also a massive strain of anti-capitalism and anti-individualism inside that group, predictably coupled with a deep sense of anger at the perceived unfairness of the modern world.
And then... You have another group that seems to prize certainty & stability, clarity of norms and social roles, and even apart from all that seems temperamentally drawn towards more "left-brained" type of work where there's a definite right and wrong way to do things.
That mentality can create other problems at the extremes, but those problems are pushing people to be too closed-minded and unable to accept differences in people's lifestyles or cultures, and in the most extreme cases conservatives will lash out against any kind of uncomfortable societal change -- ie. immigration, entrepreneurial economic disruption, etc.
What it doesn't generally do is leave the individuals themselves with a sense of personal uncertainty or depression.
So right away, I think there are major divisions in the philosophies themselves that are a root cause of progressives being more depressed.
But even outside of that, if you look at the differences in experiences between urban & rural lifestyles and the demographics of the groups that are most affected by this stuff, I think that explains a lot of it, too. Ironically, perhaps, there's a lot less obvious tension in rural areas between rich & poor, and kids who live in the country (where people tend to be more conservative) aren't inundated with billboards and other imagery stressing them out about what they have or don't have and the pressure to perform for a crowd is a lot lower.
Social media changes that to some degree, of course, but I've certainly observed that growing up in a rural area was a pretty good way to grow up less neurotic than people who grew up in cities.