As far as I know, there has been no historical study which has studied the harshness of premodern life and its effect on people's mental health, for a variety of reasons. The physical effects have been studied, I'm sure, although I don't have any expertise to comment - perhaps a medical history expert might be able to help out.

The primary reason for the lack of studies is that the 'harshness' of premodern life is largely an illusion. As you said yourself it's "from my perspective as a person living in modern times' that premodern life seems hard, but from the perspective of a medieval peasant it was normal, all they'd ever known.

In a similar vein, premodern day-to-day life was probably not as hard as it is sometimes portrayed as. Sure, you probably wouldn't have any political rights or many choices in what to do with your life, not to mention that you would be materially worse off than in the modern world. Illness, natural disasters and the attendant loss of loved ones would be ever-present threats, but the harshness of their occurrence would be no worse than they are now.

But you would probably have more leisure time than the average 21st century western person - time you could spend with friends and family, eating and drinking, playing games, telling stories and devoting to spiritual pursuits. Building on the work of anthropologists, in particular Marshall Sahlins, historians often agree that the less complex a society and its economy is, the more leisure time available to those within it: with documented simple hunter-gatherer societies working on average 3-5 hours per day and spending considerable amounts of time napping, eating and gossiping. So despite the fact that human productivity is at an all-time high and that we possess innumerable labour-saving machines, 21st century humans are on average almost certainly working longer and harder (although in less physically demanding occupations) than ever before. The current mental health epidemic may well be the result of increased awareness, but the likelihood is also that humans are more stressed than ever before too.

Some caveats to what was written above. It is very generalised, as speaking of the premodern world will always be. In different time-periods and places I'm sure that there were times when it was harsher or more brutal to live an average life. Indeed, I certainly wouldn't wish to live in any time other than the modern day west. However, I think the broad points of the answer, and the warning not to exaggerate the misery of premodern life, do hold up.

My source on leisure time and economic complexity is the hugely influential Marshal Sahlins, 'Stone Age Economics', which has been very influential on historians, particularly those working on ancient and medieval economics.