I haven't read the whole thread and I'm not qualified to comment about serious MH conditions, just the general climate of unhappiness/anxiety with young people.
I think social media has made young people's lives much more difficult over the last 20 years. Bullying, for example, now follows them home and there are so many new ways for bullies to persecute their victims. Also, 'influencers' etc present a totally false and unattainable version of life, making naive youngsters feel inadequate for ridiculous reasons.
I've noticed over my lifetime that standards of personal presentation have sky-rocketed too. Watching old TV series, the actors were much less perfect-looking - you were allowed to have funny teeth etc in the past - and women had frizzy hair etc etc. But the UK has fallen under the influence of Hollywood so much in recent years, that young women now spend time and money on emulating the supposed (ie plastic) perfection expected there.
Plus - with careers - there's the push to send so many to university when it really isn't right for them along with the growth of reality TV and celebrity culture. This has made many perfectly good careers unattractive to young people, who think they deserve a glamorous and luxurious lifestyle like those seen on TV. They feel they've failed in life if they have to do an 'ordinary' job.
Also, I think we've become a more self-centred culture - where kids obsess about themselves instead of getting out into the world in some way and making a contribution, or following an interest. I remember when labels like 'anorak' and 'sad' started to be used to sneer at anyone with a hobby (that wasn't football). and to portray them as social failures.
Loads and loads more reasons. I'm glad I was young when I was young and I feel for young people now.