Ummm ... actually, mental disorders can have a lot to do with the state of the Soul. The assumption that only one Soul animates one body is actually false - a condition called "Soul shifting" can arise from time to time. Usually this is a negative situation, in which the original Soul meant for the body gets pushed aside by a negative Soul that decides to take over. Often there is a trigger of extreme emotional distress, or drug abuse or alcoholism. I see it (and clear it) in my practice all the time.
The Soul memories are stored in the Soul's Akashic Record. But Soul situations are mirrored as psychologic problems quite frequently. You cannot separate these issues. As above, so below. The Soul shapes our incarnation very significantly - it is the driving force. Soul-level issues are manifested at the mental, emotional and physical level.
Blessings,
Andrea
Intuitive Consulting with Andrea Hess: Soul Profile and Spirit Guide Readings, Intuitive Development, Unlock Your Intuition