Suicide was an acceptable option in Greek/Roman philosophies. The 'open door' thinking is that you can check out anytime whenever the going get tough. In the last century, this thinking in the guise of personal autonomy has made a come-back. Christian thinking has always being against suicide because the right of life and death belongs to God. However I do not think that suicide is an unforgivable sin.
read these two articles and let me know what you think.