Suicide is a mortal sin and an act of defiance against the Holy Spirit, Who gives life.
Suicide is a much greater sin than murder, since for the sin of murder a man can still repent; but for the sin of suicide there is no repentance.
Here are two examples of overwhelming misfortune, in the face of which a fainthearted man would commit suicide, yet in which holy men of God showed themselves to be heroes.
St. Eustathios (Sept. 20) found himself in the following predicament: he left one of his sons on the bank of a river, while he carried the other son to the opposite bank and returned for the first son. Halfway back across the river, he saw that a lion had seized his son and carried him away. He looked at the other bank and saw a wolf seize the second son and carry him away. A fainthearted man in such a situation would have drowned himself in the water, and made an end to his life. Even though drowning in sorrow, Eustathius did not commit suicide, but with hope in God lived as a hireling for fifteen years. This patient man lived to see his two sons again. Thus, God rewarded his faith and patience.
As a young man, St. Hilarion (Sept. 20) was forced to become a Muslim, but his conscience began to torment him cruelly, and he had no peace at all. He returned to the Christian Faith, was tonsured a monk, and gave his body over to intense fasting and every difficult ascetic labor. Even so, his peace of soul did not return to him. A fainthearted man of little faith would have committed suicide. But Hilarion chose an incomparably better path. He went to Constantinople with his spiritual father Bessarion, and not only openly confessed the Christian Faith at the sultan's court, but even counseled him to go to Russia and be baptized. After being subjected to mockery and torture, this courageous young man was beheaded, and God glorified him both in heaven and on earth. Even today, his holy relics are miracle-working.
But where is the glory of those who commit suicide? Where are their relics?
St. Nikolai Velimirovich
http://agapienxristou.blogspot.ca/2013/06/st-nikolai-velimirovich-how-saints-face.html