What are 'mental illness' and 'mental health'?
If someone has always had a problem in their thinking, feeling or behaviour, then this is not usually called mental illness. It may a developmental problem or a difficulty with their personality (sometimes called a personality disorder).
Mental health is the opposite - it means mental wellbeing, good mental functioning or having no particular problems in thinking, feelings or behaviour.
These definitions over-simplify things. We all experience changes in our feelings, thinking and behaviour, and there is no clear cut off between illness and health. Also someone may have problems which fit the definition of a mental illness, but be very healthy mentally in other ways.
Mental health problems can affect any of us at any time in our lives, so there's no need to be embarrassed about asking for help. Looking after our mental health is just as important as looking after our physical wellbeing.