God can only present His promises to us. He will not supersede our own choices. I agree that God is always there for us, but there are times in life when that simply isn’t enough for everyone.
Again, this isn’t God’s failing. It’s ours. Do keep in mind, we are not perfect. We can try to live good lives, and love God, and respect Him, but there can be times when the human condition is too weak to prevail.
None the less, I do see that failing in this context to be the unpardonable sin.
Romans 8: 28 & 29 leave no room for human error. Our sin abounds but grace abounds more. I can’t say whether or not a true born again Christian is capable of suicide, but I can say a Christian who has a dynamic relationship with Christ is incapable of suicide.