I don't believe that there are any unforgiveable sins. A dozen people might commit suicide and there may be at least 12 reasons, if not more, for their decisions. Unless we are able to read minds, we do not know if these suicides were sins or not and only God knows if any sins are forgiven. Trying to answer this question will be as impossible of a dog chasing its tail with the hope of catching it!
I have become more familiar with deaths by suicide than I ever imagined I would. During the five years I served as a police department chaplain, I responded to dozens of suicides and experienced the pain of families who couldn't imagine a loved one ending her or his own life. Parents turned to me often and asked if God would forgive their son or daughter for this sin. I could only explain that I believe in a loving and forgiving God. You and I do not know what was in the heart and mind of the deceased. We do not know what motivated the actions that caused their deaths. The only answers we have are questions.
Some of my mortician colleagues have a horrible time dealing with the bodies of those who have committed suicide. Who are we to judge? So anguished by a murder spree where he killed five people in two separate incidents, a young man was sentenced to life in prison. Early in his confinement, he was found hanging from a sheet in his cell. The funeral director who would be responsible for his services wanted nothing to do with the body. Not only had this young man killed five innocent, but he killed himself! Certainly, he did not hang himself for the pleasure of suffocating. Material found in his jail cell indicated that he had tremendous remorse for his actions and that he felt he could not live with that any longer; he had taken lives and now his too must end.
Nothing in the teachings of Jesus are you and I told to judge another's deeds and actions. In fact, Jesus teaches that we cannot judge another until we remove our own sins. Although I believe that suicide is a selfish act, I do not believe it to be either unforgiveable or necessarily a sin.
Let us say that a 35 yr. old woman, married with three children, finds that her husband has been having an affair with a younger woman and plans to divorce in order to marry the other woman. She turns to her mother for answers and advise and is basically told to "suck it up." This woman has only worked simple jobs and those early in her 15 year marriage. She had begun college but quit when she married and she felt she had no useable skills. One child was born with a congenital spinal condition which required multiple surgeries and orthopedic devices and the expenses would be impossible to sustain without the insurance her husband had for the family (and would probably drop with the divorce). He had no intention of taking the children into his new family and had already said that he would do everything in his power to not pay "a dime" toward care for the family.
Day after day; week after week, this woman was faced with the reality that she would soon legally and physically be alone with three children, no income, and bills piled high on her desk. She turned to alcohol for consolation and found none. The alcohol in fact, simply complicated her depression.
One evening, after she had gotten the children off to bed, this woman decided that she could not handle the stress and uncertainty. She took an overdose of Vicadin along with a half bottle of Vodka and stopped breathing. She was found about six hours later by one of the children who called 911 but resuscitation was not possible.
Was this suicide a sin? If so, was it forgivable or not? How can you or I judge that? Tomorrow, some major happening might throw us into similar emotional turmoil and despair. How will be handle that stress? Would it be an unforgiveable sin if we decided to end our lives?
I believe that the unforgiveable sins associated with suicide are actions such as forbidding burial in "consecrated ground." That dead human body is not capable of sin. The essence of that person's being, is gone to another realm. I believe it is somehow united with a divine presence and that any concerns regarding the suicide are being taken care of in a plane other than a grave in a cemetery. This action by a Church body only causes additional pain for a family, a community, a parish.
We so frequently use one word terms for complex actions. Suicide is so very complex and individual. I am certainly not ready to judge suicide an unforgiveable sin and hope that the rest of society will wake up and reconsider the condemnations and disgust many associate with suicide.