i once read that some thought the unpardonable sin might include grieving the Holy Spirit in the sense that a believer attribute an action by The Lord as being from the enemy or of evil intent. Hope not - i am guilty of that myself. Perhaps it means turning your back on Him, breaking his heart. I hope I haven’t done that. Though Scripture does seem to say that once saved, always saved otherwise. The Lord is extremely patient and forgiving - He is indeed a true friend who shows true love to us.
In suicide, however, a Believer not only ends his own mortal (physical) life but he also kills that part of the Holy Spirit that resided inside that Believer, which may be why it is unpardonable. And yet I have felt depression to the point of wanting death and find it hard to condemn others who succumb to that ultimate temptation.
God ministers to us through our suffering and suffering not only allows us to minister to others but it also gives us "street cred" when we tell others they can overcome their circumstances with prayer and faith. God loves "hopeless causes" because then He gets to flex His muscles and demonstrate His amazing powers. He took the Apostle Paul from the most virulent of Christian haters and made Him the leader of His gospel ministry - because He can.
If committing suicide isn't the "unpardonable sin", then the only other thing I think it could be is to publicly renounce your faith and salvation.
If it isn't one of those two things, I believe a lot of us are in heap big trouble.