I believe that mental illness what just as prevalent in history as it is today. The difference was in how it was handled. Two things come to my mind: denial ('so-and-so's mom is always sick, she just lies in bed all the time'...the man who comes home from work every evening and has three cocktails 'to relax') and that nearly every family had at least one member who was quietly 'put away' (institutionalized). In my own family, on my father's side my great-aunt spent 30+ years as an inpatient. The family didn't talk about it and only occasionally, quietly went to visit her. On my mother's side a cousin was institutionalized for two years after her baby died from a childhood illness. The poor women had a 'nervous breakdown' (what we would now call a psychotic break) and was 'sent away'.

Just look at all the 'asylums' that existed, and were full of patients. Mental illness was so feared, so taboo, people just didn't talk openly about it.