I think you also have to look at how social pressures or worries have changed and why that means things are different now.
In the past people would have been worried about invasion by the Normans, crop failure, plague, being accused of witchcraft and burned at the stake, hunger, being cold, divorce was a stigma so was being an unmarried mother, women would have been completely at the mercy of cruel husbands, women were subjected to awful treatment - not allowed to own property and subjected to double standards about sex (still are to an extent), then people worried about conscription, the men of their families being killed in the trenches, then if their houses would be bombed. In older times if people were poor, life was very tough and they had to go without - like when children went without shoes or had to leave school early to work.
Nowadays those things aren't really relevant but there are problems that didn't exist then - social media, online bullying, debt through credit cards, car payments etc, there is so much more "stuff" available for people to buy and feel they need, there are issues over gender and sexuality which couldn't have been acknowledged before, there are more single parents because people go their separate ways more now. Jobs are unstable and in short supply. It's more common to be a home owner in modern times and house prices are out of many people's reach.
But then there are issues which have always been the case - self esteem, poverty, illness, bereavement, job loss, crime, infidelity, domestic abuse.
I would really like to know what the true instance of MH issues has been historically but that would be very difficult to find out because of the stigma and lack of access to appropriate healthcare in the past.