Let me be blunt: Your brain is telling you lies

Mental illness is a physical disability, not mental. It has mental effects, most certainly but that is a symptom of a larger whole.

Imagine you had a slipped cartilage in your knee. You walk, it moves and you suffer. Would you say "why am I not able to walk normally? I am walking correctly?" No, you would not. Doesn't make sense

Mental illness has a very long history of being treated as a personal failing, a weakness in the person, not an illness. Which it is

If bad events were the cause, there are people born into horrific poverty, growing up in refugee camps, hiding from murderous guerrillas in the night who would not have a genuinely cheerful disposition (not me, I know someone who lived that)

The dichotomy of mind and body as separate entities is not true. Depression is not an illness of the ghost in the machine, it is an illness of the brain, an organ, which has loads of additional effects