Research.

Don't strain yourself on your own knowledge. It is better to heighten what you know so you could write more. 

For example, when give a topic of a psychological illness like Multiple Personality disorder, read about it. Don't worry about how many tabs you open just read to your heart's content!

In this way, you wont feel limited and few of words because you know about the topic so much that you could probalby write one long chapter all about it. 

As for me, I picked  Multiple Personality disorder (or Disassociative Identity disorder) it is a psychiatric disorder characterized by having at least one "alter" personality that controls behavior. The "alters" are said to occur spontaneously and involuntarily, and function more or less independently of each other. The unity of consciousness, by which we identify our selves, is said to be absent in MPD. Another symptom of MPD is significant amnesia which can't be explained by ordinary forgetfulness. 

Memory and other aspects of consciousness are said to be divided up among "alters" in the MPD. The number of "alters" identified by various therapists ranges from several to tens to hundreds. There are even some reports of several thousand identities dwelling in one person. There does not seem to be any consensus among therapists as to what an "alter" is. Yet, there is general agreement that the cause of MPD is repressed memories of childhood ual abuse. The evidence for this claim has been challenged, however, and there are very few reported cases of MPD afflicting children.

source: skepdic

You'll get a lot of ideas if you research :)