Over the years I have found it more useful to limit the term mental illness to severe psychiatric disorders only. By these I mean disorders such as schizophrenia, bipolar one mood disorders, and severe major depressive disorders. I do believe most forms of adult ADHD are simply variations on human experience and neurocognitive processing style. if unaddressed and mismanaged in youth, many forms of ADHD can morph into more severe forms of mood or anxiety disorders in adulthood. If one learns how to manage and harvest the beneficial aspects of ADHD such as problem-solving, hyper focusing, intuitiveness, creativity, atypical time management techniques, etc. then what we refer to as adult ADHD could be a model for  neurocognitive styles.  These styles can  be managed for completion of certain tasks that require sustained attention. Treatment then is focused on managing problematic aspects and symptoms specific to various contexts and not a global approach as an illness!. This has huge implications in self perception.