There are a lot of conditions defined as mental illness but the worst of these is SELF DOUBT... It is absolutely the worst. Doubt slays our best thoughts. Doubt splits our mind in two and puts us into conflict with ourselves and then with others..
If doubt is not resolved quickly, it leads to all kinds of undesirable behavior such as becoming obsessive or compulsive, manic or depressed or developing an ADD type personality or becoming addicted to foods, booze or pills as a way to squash the pain caused by doubt.
If there is such a thing as a devil, his real name is Doubt.
If you leave your doubts alone, they will leave you alone. However caution and vigiliance is called for here. Doubt is the great tempter and it will do whatever it can to make you doubt yourself, your mental health or your decisions. If your doubts aren't throwing you into conflict with yourself, causing you unnecessary pain and suffering, the doubts of others will.
If you don't believe doubt is your worst enemy and the root cause of mental illness, watch how many times your doubts betray you....