IMO, Mental illness is actually physical illness manifested in the brain and personality.
I struggled for years with my father's suicide. Dad wasn't obviously or overtly "mentally ill" but was suffering from depression, nobody else in the family knew to what extent.
I got a greater understanding and a degree of acceptance when I was confronted with such physical pain of my own that I preferred death. Not that I contemplated suicide but did indeed prefer to be dead rather than suffer any more. It hit me like a ton of bricks that there are people who suffer mentally to that extent.
I'm not justifying suicide but trying to better understanding it.