If ѕυιcιdє is "mitigated" due to mental illness, then exactly what ѕυιcιdє is not mitigated?  Do we really believe that a sane person would take his own life?  

And if all ѕυιcιdєs are due to mental illness and therefore should be mitigated, then doesn't that also mean that the Catholic Church taught error for hundreds of years?

The Catholic church has never taught error. What happens sometimes is that She will recognize the mitigating circuмstances that may have an effect on the culpability on what is the grave matter that makes for a mortal sin.  

And the simple reason why ѕυιcιdєs go to Hell is because ѕυιcιdє is a most grievous mortal sin against the Holy Ghost which cannot be repented.

Where do souls, who are in the state of mortal sin at the time of death, go?

St. Thomas on Whether it is lawful to kill oneself?

It is altogether unlawful to kill oneself, for three reasons. First, because everything naturally loves itself, the result being that everything naturally keeps itself in being, and resists corruptions so far as it can. Wherefore ѕυιcιdє is contrary to the inclination of nature, and to charity whereby every man should love himself. Hence ѕυιcιdє is always a mortal sin, as being contrary to the natural law and to charity. Secondly, because every part, as such, belongs to the whole. Now every man is part of the community, and so, as such, he belongs to the community. Hence by killing himself he injures the community, as the Philosopher declares (Ethic. v, 11). Thirdly, because life is God's gift to man, and is subject to His power, Who kills and makes to live. Hence whoever takes his own life, sins against God, even as he who kills another's slave, sins against that slave's master, and as he who usurps to himself judgment of a matter not entrusted to him. For it belongs to God alone to pronounce sentence of death and life, according to Deuteronomy 32:39, "I will kill and I will make to live."

So why did the Church not make any exceptions for alleged "mental illness" until just recently?

It seems that just about every single depravity and atrocity nowadays can be explained as a case of "mental illness". Sodomites, transgenders and rapists are examples of mentally ill people. Do their sin is somehow lessened because of their deranged mental condition?

My cousin committed ѕυιcιdє but mental health was an issue.  Apparently he had multiple concussions from sports in high school and it made him have a lot of mental health issues.  So his brain was sent of to a university for testing and he did have severe CTE.  I do not know how real CTE is but I have not heard anyone claiming it does not exist.  My sspx priest told me I could go to his novus ordo funeral, I told him what I knew at the time, which was that he had concussions and that was the suspected cause of his mental health issues.  He grew up in a upper middle class household, going to expensive private novus ordo schools, and lived a normal life.  Most likely he never returned to the faith, never giving a confession before he died.  I most likely assume he had premarital sex etc, but he was a baptized catholic, who killed himself but what fault was truly his?...  
I am totally against ѕυιcιdє, I would bet my life savings he is not in heaven, but I do not know for sure.  ѕυιcιdє in the modern age is very tricky and complicated.  

So long story short, I went to his funeral, which had over a thousand people attend (his father had a lot of business associates).  There were 3 or 4 novus ordo priests at the "service"; at the "sermon" the "priest" said my cousin was in heaven and he had left his pain in this world.  He said he was in heaven multiple times.  We left the service and waited until the end to go the a luncheon at he church.  

Did I do the right thing?  I do not know, my priest told me I could go, and I do not know for a certainty he was not insane, or not in sanctifying grace.  

I think you did the right thing to go to his funeral. I think the right thing would be to pray for the repose of his soul. 

I think a person who has no faith, if he has CTE, on reading this,   https://www.protectthebrain.org/Brain-Injury-Research/What-is-CTE-.asp
Would be sorely tempted to commit ѕυιcιdє, Especially in a world where perfect health, beauty, sporting prowess, intelligence etc are the non-plus-ultra which must be attained, and there are no other values worth seeking after, like unity with God.
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THey are certainly not thinking for Eternity.
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THis is what it sounds like in the case you mentioned tdrev. Another factor, I think, is that parents don't raise their kids by preparing them for the hard knocks, so when the hard knocks come, and they do, the kids are traumatised by them and don!t cope well.
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Once upon a time some years ago the doctor would have brought in some rattles to try to cure you of that nasty cold you had. Another time the doctor would have prescribed cigarettes for your cough. 

Once upon a time some years ago the doctor would have brought in some rattles to try to cure you of that nasty cold you had. Another time the doctor would have prescribed cigarettes for your cough.

What on earth are you rambling on about?  
Whatever it is, it is totally unrelated to anything I have said in my post.

I've known more than one person who has not committed ѕυιcιdє just for the fact it'll end badly (Hell).

Antipsychotic drugs given for depression cause depression and suicidal thoughts. Read the list of  side effects of these drugs.

Even if a person knows that ѕυιcιdє leads to hell, their thinking can be distorted by these satanic psychoactive drugs that fossilize the pineal gland, the center of our brain that enables us to pray and to have communion with God. Look at the dangerous chemicals in our pharmacies. Even antibiotics, like Cipro and Xithromax, contain fluoride, which is one of the most dangerous chemicals on earth.

Why else did Japan and Germany fluoridate the water during WWII? Why did Obama force all major U.S. cities across the USA to begin fluoridation? Water fluoridation causes people to become compliant like sheep. It causes people to lose the ability to discern or to think rationally. Look at the numbers of people throughout the USA who have abandoned the faith. Fluoride causes people to lose the faith.

With increasing numbers of fluoridated drugs on the market, and with perhaps more than 70 percent of the drugs in the pharmacy containing fluoride, we are all at risk whenever we visit an M.D. because our medical doctors have become drug pushers for Big Pharma.

ѕυιcιdє is OBJECTIVELY a sin against the Holy Ghost.  And the Church presumes, in the external forum, that the soul has been lost.  But there are other requirements for mortal sin in addition to grave matter, and psychological illness could possibly have mitigated his culpability.

Not a psychological illness, but a neurological disorder would mitigate culpability.