Mental illness is best described as a medical condition that affects thinking, feeling, or mood. When your ability to cope with the demands of daily life becomes disrupted, you may be diagnosed with a mental illness, of which there are many different kinds (depression, anxiety, schizophrenia, bipolar disorder, obsessive-compulsive disorder, etc.). Trying to deal with a mental illness can be emotional, confusing, and isolating; and can prevent you from living a full, happy, and successful life. But as damaging as they can be, mental illnesses are treatable.