SUICIDE: IS IT UNFORGIVEABLE CAUSING ETERNAL DAMNATION: NO!
If you are a believer then all sins are paid for and forgiven, past, present and future relative to eternal life even future sins - even suicide, murder, etc. but there are severe consequences for the child of God who sins and takes himself out of fellowship with God for extended periods of time: especially for one who cuts short his own life and opts out of doing the works that God has set before him to do - consequences that that individual will find far worse than facing whatever he has to face in this mortal life. There are a number of passages that teach the eternal security of the believer, for example, once saved one is sealed by the Holy Spirit as an absolute promise of God's that you will go to heaven no matter what, (Eph 1:13-14).
BUT the consequences of sinning for the believer are awesome and to be avoided at all costs: loss of eternal rewards when sin takes one out of God's will for extended periods of time can lead depending upon the severity and amount of sinning to years and years of weeping and gnashing of teeth due to the loss of opportunities in the millennial kingdom of the future. This mortal life will only be for a short while compared to the 1000 years one will have to suffer loss in the future kingdom and then for the rest of eternity: lesser capacity to enjoy one's eternity due to the loss of being given that capacity for staying in there and serving the Lord.
II) CONSEQUENCES OF SIN FOR THE BELIEVER
A) TEMPORAL JUDGMENT AND DISCIPLINE
Heb 12:5-11 - severe chastening, physical incapacity, sickness, death
Phil 4:6-7 - loss of peace
Heb 13:18 - guilty conscience
Ps 32:1-11 - sorrow
I Jn 5:16-17 - death
I Cor 11:28-30 - sickness & death
Pr 14:27 - death
Pr 11:19 - death
Ez 18:4, 20 - death
Jas 1:15 - death
James never calls his readers to salvation but many times he calls his readers to give up their worldly ways.
Cp.1:21 - note that this verse contains the phrase "to save your souls", (psuchas = the soul as an essence which differs from the body and is not dissolved by death, more specifically in this context the life which is lived on earth will be saved if the believer turns from living a sinful life and thereby save his physical life from being cut short before his time. James is telling his readers that believers must give up their worldly ways in order to save themselves from losing their life on earth - from physical death which includes the cutting short of the time the soul spends on earth in the physical body doing the Lord's work; for the soul departs from the body upon physical death. If nothing was produced by that soul while in the physical body due to a sinful lifestyle then that soul has wasted himself and will have nothing to show for his time on earth - his life will have been worthless. Nothing he did will be saved in heaven for all eternity!
Compare other passages in Scripture which deal with the subject of death of a believer due to a persistently sinful life:
1 Cor 8:9-11:
A Christian perishes, (dies), due to a weak conscience and his consequent sin.
1 Cor 11:28-30
A believer dies physically because he violates the sharing in the Lord's supper with continuous serious mental attitude sin.
1 Jn 5:16-17:
The sin that leads to physical death IN A BELIEVER is indicated as a truth
A husband and his wife die due to sin
The fear of the Lord adds length to life, but the years of the wicked are cut short.
The truly righteous man, (the believer who maintain temporal fellowship) attains life, but he who pursues evil, (could be believer or unbeliever) goes to his death.
Sin is death dealing. Every card it deals says "death" on it. To play in the card game of sin is to invite one's own death. The more one sins the closer his death approaches and the more miserable his experience becomes.
The obedient Christian will experience inner joy and peace (Gal 5:22-23). The disobedient Christian will not.
B) GREAT LOSS OR GAIN OF ETERNAL REWARDS DEPENDING UPON THE FAITHFULNESS OR LACK THEREOF OF THE CHILD OF GOD
3) LOSS OF SALVATION IS NEVER IN VIEW FOR THE CHILD OF GOD
III) CONSEQUENCES OF SIN FOR THE UNBELIEVER
A) ALL MANKIND'S SINS ARE PAID FOR AND BUT ONLY THOSE WHO HAVE TRUSTED ALONE IN CHRIST ALONE HAVE ALL OF THEIR SINS FORGIVEN PAST PRESENT AND FUTURE
D) THE GREATER THE AMOUNT OF SINNING THE GREATER THE PUNISHMENT IN THE LAKE OF FIRE