It is important to make a clarification. This is not common chemical imbalances that can be solved with proper implementation of elements. Extreme mental illness is beyond what we call the mind and can only be solved by an individual with the power of mediumship. Examples of Mediums include Evangel, Bodhisattva and Messiah. One with an internal full spectrum light emitting from a fully active chakra center. One with no fear of death as one must die continuously in order to lock onto the frequency of the lost.
To elaborate a little further, their auric field is unbalanced due to an entity leeching from it - simultaneously throwing the chakras off and thus the correspondent organs offline. This is called a "rider" - giving us terms like "monkey on your back". It is in fact an ancestor on your back. A real medium is capable of using their body as a portal. The lost can leave the gaseous unfixed state of the netherworld for a moment of much more fixed third density to gain light and energy. If the two are cohesive, they can then be given clearance which simultaneously does the same for whatever being they are cleaving to thus causing some addiction, infirmity, or insanity.
Why would an ancestor cause such troubles? it is merely the ancestor's attempt to act out one of their favored moments one more time through someone they have a bio-rythmic connection with. This could range from simple quirks to extreme mental illness and abusiveness. Notice that children inherit various traits from their parents, in turn the parents inherit the traits from their parents and so on. This is the ancestral connection. Often indigenous tribes evoke ancestors through gestures that seem peculiar to the onlooker but are in fact the movements that "attract" ancestral spirits based on their familiarity, hence the term, familiar spirits.
Activated humans fulfill the call for Neo-Mediums amongst many other things, guiding their loved ones looping in the qlippoth of stasis to the great beyond, thus ending meaningless incarnations.