I don’t think anyone here understand what is being mentally ill, so before any mention of who is mentally ill allow me to explain that nobody has mental disorders or illnesses.
Firstly mental illness is a term used by psychiatrists that shouldn't be used. The mind is something that can't be seen, has no smell or taste. The brain on the other hand does, but there is little neurology can understand about the brain and mind without psychological interpretation. But how can the mind have an illness? This is just as ridiculous as saying that the soul has an illness.
The other problem is that psychiatry has set the line for normality, to determine abnormal psychology, sadly as some tautology of theirs with little evidential support. Saving Normal is a book written by the person responsible for the DSM-IV, a past bible used by all psychiatrists, which (in the book Saving Normal) he explains the issues with normality. Some clinical psychologists (The New Laws of Psychology and The Book of Woe books) have gone so far to explain that there is no such thing as abnormal or normal psychology, just psychology.
The other problem is of what the layperson thinks of mental illnesses and mental disorders. As if irrational behaviours explain some illness. Let me here explain briefly what is going on with JW’s. People are somewhat born as blank slats, and jws implement a system to control and monitor a members level of education and socialisations in such a way that makes it difficult for differentiation dialogue to take place to make way for change. The other problems are human problems, cognitive dissonance (- Mistakes Were Made (But Not by Me): Why We Justify Foolish Beliefs, Bad Decisions, and Hurtful Acts – is an excellent book on this), cognitive biases (The Art of Thinking Clear) and other cognitive distortions and heurists (my favourite book Thinking, Fast and Slow). All this books explain how we think we are rational beings when in fact we are very irrational.