It's always easy to point the finger at a single cause, but the reality is that it's a series of factors that together act like a sophisticated, compound "attack" to the psyche:
- Social media weakens your self-confidence and makes you feel in a constant state of inferiority, i.e. your present self.
- The "post 9/11 world" you mention, is simply a result of a general worsening of Western political relations over time, which are becoming especially evident recently. That state of turmoil weakens your sense of stability and security, making the future look bleak - i.e. your future self.
- Further blows for the knockout: climate change, pollution / micro-plastics / rising costs of living, etc.
I'm actually surprised depression, anxiety and other mental disorders aren't more common...but perhaps they will be.