Replies

For consideration in-depth, see Suicide in the Catholic Encyclopedia. An insert on the top of the old article contains a quote from the today's Catechism:

"Everyone is responsible for his life before God who has given it to him. It is God who remains the sovereign Master of life. We are obliged to accept life gratefully and preserve it for his honor and the salvation of our souls. We are stewards, not owners, of the life God has entrusted to us. It is not ours to dispose of."
"Suicide contradicts the natural inclination of the human being to preserve and perpetuate his life. It is gravely contrary to the just love of self. It likewise offends love of neighbor because it unjustly breaks the ties of solidarity with family, nation, and other human societies to which we continue to have obligations. Suicide is contrary to love for the living God."
"If suicide is committed with the intention of setting an example, especially to the young, it also takes on the gravity of scandal."
"Voluntary co-operation in suicide is contrary to the moral law."
"Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide."
"We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. The Church prays for persons who have taken their own lives."
(Paragraphs 2280-2283)

There are also exceptions. Martyrdom is refusal to betray Christian faith in the face of death; a related scenario is heroic act of war, leading to death of the hero. These, since the death is not the desired outcome but merely a price for the outcome that is virtuous, are not suicide.

Mental illness is an excuse of many sins and it excuses this one, if the victim cannot control his actions.

During the Civil War in Russia the Russian Orthodox Church would excuse suicide of soldiers that killed themselves in order to to be captured by the Red Army and endure certain torture. They were given proper last rites. That was not a dogmatic decision but rather a judgement by the clergy attached to the White units, who knew what the men were facing.