Suicide is a sin because it destroys someone who was created in the image of God. But nowhere do the Scriptures say that suicide is a sin that is not forgivable.
Suicide is perhaps one of the most devastating acts that someone can do. It puts an end to a life that God began--and it transfers anguish and pain to loved ones for the rest of their lives. But the Scriptures are clear that nothing that happens in life or death can separate those who are in Christ from Christ (Romans 8:28).