Friend,

Calvin writes about Hebrews 6:4-6:

In short, the Apostle warns us, that repentance is not at the will of man, but that it is given by God to those only who have not wholly fallen away from the faith. It is a warning very necessary to us, lest by often delaying until tomorrow, we should alienate ourselves more and more from God. The ungodly indeed deceive themselves by such sayings as this, -- that it will be sufficient for them to repent of their wicked life at their last breath. But when they come to die, the dire torments of conscience which they suffer, prove to them that the conversion of man is not an ordinary work. As then the Lord promises pardon to none but to those who repent of their iniquity, it is no wonder that they perish who either through despair or contempt, rush on in their obstinacy into destruction. But when any one rises up again after falling, we may hence conclude that he had not been guilty of defection, however grievously he may have sinned.
(my emphasis)


Note that last sentence. If David could be forgiven of his sins of adultery and murder, it would seem to me that you are not beyond the reach of God's grace. But may the Lord grant you a truly contrite heart, and a right spirit within you.

In Christ

Theo