Suicide is a sin because the bible says God made man in His image and God wants to take us in His timing, not ours. Taking our own life means we have lost all hope because we have our eyes on our circumstances and not on God. God wants us to bear our burdens so He can work thru us and be witnesses for Him for others to see His goodness and mercy. I have been on several death beds, hospitals, jails, and believe me if God had never heard and answered my prayer for mercy, I would not be writing this today. Some of those experiences did make me even consider suicide they wee so bad, but I am thankful that my parents were Christian and took me to church where I learned that Jesus loved me as a child. So I understand how dark life can be. I believe a saved person still goes to heaven, but it is not what they should be doing. They will probably lose all their rewards, but it is God who makes that final judgment because He knows all our circumstances and All things are possible with Him.
Remember that suicide is a sin, and there is a penalty for sin. "The wages of sin is death". Wages are something you earn, and Jesus has paid that debt for those who believe in Him. So the unsaved person will go to hell because they can not pay for that sin. The ONLY one that can do that is Jesus Christ and you cannot "accept the gift of God for eternal life" once you are dead. The bible clearly says in John 14:6,
"Jesus saith to him, I am the way, the truth and the life: no man cometh to the Father, but by me".
Lastly, besides a being serious sin, suicide is wrong because everyone who loved you or cared for you will probably carry the burden of guilt for the rest of their life because they will feel responsible they might have prevented it. I remember years ago, I heard a gunshot and a woman scream. They had some argument and she wanted to break up. Her boyfriend shot himself in the head in front of her. Do you think she will ever get over that? Here is another story about suicide.
Years ago, I was sentenced to jail for six months for a crime I didn't do. It was a small town in Central Calif and I was out on bail pending an appeal and driving back to LA. I was very upset, so I stopped and picked up a hitchhiker so I could have someone to talk with. The moment he got into the car and I looked into his eyes, I knew I had made a mistake picking him up. But even though I was afraid of him, I kept on driving and I told him all about my situation. After a short drive, he wanted to stop at a restroom and when he got out and went in, I could have left him but I didn't. I hid my money and continued on.
When I finished my story, he trusted me because we had something in common as he was a convicted felon. He said had just gotten out of Soledad prison for strong armed robbery. Prior to his conviction, He said that his brother had committed suicide when a cocktail waitress he had introduced him to, broke off the relationship. He felt very bad and he had gone to a priest and the *priest told him that suicide was an unpardonable sin and his brother went to hell. That heavy guilt trip ruined this man's life because it made him turn to a life of crime. This happened over thirty years ago, and I have never forgotten it, and if he is still living he probably hasn‘t either.