Please feel free to ignore this or even shoot me down in flames, I don't want to be alarmist, but it doesn't sound completely fanciful to me. Some people are more senstive than others.You work as a plumber, so you must have been exposed to a fair amount of lead in old pipework and solder? Also you have lived in California where the air pollution can be bad and you are of an age where petrol (gas) used to have lead added to make the engines run smoother. I think I've read about toxic waste in the woods in Michigan too, so you could have had a pretty high lifetime exposure to lead. Symptoms of lead poisoning can include depression and mood disturbances. Lead Poisoning | Doctor | Patient.co.uk

Another possible source is the mecury amalgam used in some tooth fillings, what is your dental work like? I once read about tooth fillings causing all sorts of long term health problems. The only reliable science I could find was inconclusive, so I think my level of concern on that one is more about my own personal anxieties.

Slightly off topic, I'm interested in the subject of heavy metal toxicity as it has been used as a theory to explain the long term decline in serious crime in the developed world. Particularly linking the drop in delinquency to the switch to unleaded petrol in the 1990s.

To me, heavy metal toxicity in someone with known exposure and already predisposed to mental illness seems like a theory worth investigating if only to rule it out as an aggravating factor.