Why would Trump cause depression? First of all he basically said all the things young people care about (climate change, environment, social justice etc.) are nonsense. Secondly his "diplomacy" was pretty aggressive. None of us non-US citizens wants to get drawn into a (possibly nuclear) war. Thirdly he caused a very big divide, not only in the US population but worldwide. Everything has become black and white. You're (as seen from the proponents of the other side) either a weak woke justice warrior soy boy snowflake or an old white racist Nazi hillbilly sociopath. There is no "gray" zone.
In addition to that the divide, his "free speech" approach made many topics that were previously taboo (also outside the US) daily topics again. Extreme nationalists but also extreme antifascists dared to speak out loudly again worldwide. Their extreme plans cause anxiety and stress.