Something about THC is at the center of this. And also something about age.
I used to be able to smoke as much as I wanted and just enjoy it, then one day it started making me crazy.. and I kept smoking for a few years and truly lost my mind.
I've heard this story countless times... something (usually around age 20) just snaps, and suddenly THC == paranoia.
Also to note, psycho active drugs are as they are called: psycho active. With prolonged use, tolerance increase, with further use some imbalance occurs especially when not under the influence of that substance. Re establishing that balance takes time. We are very poorly educated when it comes to mental stability, and pharmaceutical treatment is the de facto answer by medical corps.
I do hope you get better, just don't forget substance use has an impact on the brain subtle chemistry, that's in a way what makes them attractive in the first place, and this impact doesn't disappear after a day like a hangover. Take some time off stimilant and psychedelics, you will slowly regain your normal self.
It's common for people to smoke more and more cannabis as their tolerance grows, and higher doses of cannabis can easily lead to paranoia and other adverse mental effects.
If these people took a long break and then tried the smallest amount of the weakest pot they could get, they might find that they can enjoy it again without getting paranoid.
And in your first sentence, do you mean you get psychosis or hallucinations from smoking pot? Or literal schizophrenia? Interestingly, around 19-22 years old when I smoked a lot, I had some psychosis-like symptoms, mostly auditory hallucinations. They weren't very disruptive or paranoid-feeling though, it was always the sound of a group of people at a party or backyard barbecue or something standing around laughing & talking, and I could only actually make out like 1/5 words. It felt very real though, I looked out my window countless times to see if my neighbors were having a 2am party in their back yard.