Hi caringmum. Welcome to Psych Central. The causes of mental illness are complex and can include genetic and environmental influences. There are no clear guidelines that connects the dots easily.

One thing to consider is that people of previous generations were taught to "put your nose to the grindstone" and "tough it out" neither which I advocate. But many times people found ways to cope with mental illness because they had to. I have relatives and friends that will attest to this. Now early testing and treatment is available. Look at all the new diseases and treatments that are arriving on scene in the last few years.

Here is more detailed info
https://psychcentral.com/disorders/b...sorder-causes/

https://psychcentral.com/lib/living-...olar-disorder/

https://psychcentral.com/lib/bipolar...nes-uncovered/