We probably have a translation problem here. In Italian Mental Illness is a very broad term, comprising derangement and both Neurological dysfunction and damage. The first category, broadly speaking, is what I was talking about and you'll hardly find a clear case of the mythical "borderline" (the origin of the term comes from this concept) dividing a sane mind from a deranged one.
Derangements (eg: PTSD and Phobia but according to some authors even some forms of Addiction and intollerance ) are the results of the interaction between an (otherwise "sane") mind and extremely difficult "environmental" circumstances.
Unable to solve the situation using normal cogitation and instinct, a PTSD victim for example will react to otherwise harmelss stimuli just because that's how his mind learned to survive to those extreme circumstances...
Then you have neurological dysfunction or damage, the only difference between those two is the source. Usually we call dysfunctional a brain that was born with defects, while damage is subject to external sources. In any case, those are the situations where sections or whole structures of the brain ceased/reduced/altered their function, their chemistry. And this is another can of worms entirely, the brain is so complex that we can't even have a fucntioning model of it, that means in order to study a particular area of the brain or the differences between a functional area and the same are in a dysfucntional brain we need to wait for someone to show up with the right kind of damage/birth defect. (<-- ideally and morally)
neurological disorder is usually very painless.
Err.... yes and no. Disorders is a broad word that just means out-of-balance. If being out of balance is due to a birth defect or a damage at a very young age sure, you'll have little to no understanding of what's going on. But in the majority of cases a more mature "normal" mind is able to perceive the signs of degeneration if it happens gradually enough. And that's a fate I'd never wish upon my worse enemies, bro! I suppose you've never known someone who's been diagnosed Alzheimer at a relatively young age, I have and I assure you it's... horrible.
Shitposting aside, I'm a psychologist. My main field of work is development and learning (I mainly work as a teacher) but I know what I'm talking about. Unfortunately I also have some first hand experience, due to bad shit happening in my family. I personally saw the mental decline of my father before his death and I assure you, he knew what was happening and suffered
a lot from knowing...
Sorry for typos, grammar and if I misused some terms. Writing from my phone and I don't know if some of the words I used to define neurological alterations carry the same meaning... Probably scientific magazine would use different terms but I hope you got the gist of it.