Indeed it would be a crazy world. Post traumatic stress disorder would probably rate as the number one disorder, followed closely by depression and various forms of anxiety. Apart from the more interesting issues around someone suddenly coming of their antipsychotic medication there can also be some nasty issues with suddenly stopping antidepressants too. For example, ceasing a number of them will leave someone at high risk of suicide within the first 3 weeks.
Schizophrenia is a genetic condition so rates of it's occurrence in the general population shouldn't be effected, though other forms of psychosis could be increased. Everyone is capable of becoming psychotic and the risk of it occurring increases with lack of sleep, poor nutrition, head trauma, stress, and drug and alcohol abuse. I'd think that all the things I just mentioned would be pretty common experiences in the Twilight world (How many people cuddle up to the still on a cold night and think, "One cup for the jeep, one cup for me")
