Your reference to Hebrew 5:6 confuses me. Are you talking about these verses?
" For it is impossible for those who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost,  Have moreover tasted the good word of God, and the powers of the world to come,  And are fallen away: to be renewed again to penance, crucifying again to themselves the Son of God, and making him a mockery." (Hebrews 6:4-6)
In this case, see Bishop Challoner's explanation on what St. Paul means by the phrase "it is impossible":
 "It is impossible": The meaning is, that it is impossible for such as have fallen after baptism, to be again baptized; and very hard for such as have apostatized from the faith, after having received many graces, to return again to the happy state from which they fell.
You seem to be confusing this with the unforgivable sin, the blasphemy of the Holy Ghost. See Bishop Challoner's note on what this particular kind sin consists of (emphasis mine):
"Therefore I say to you: Every sin and blasphemy shall be forgiven men, but the blasphemy of the Spirit shall not be forgiven." (Matthew 12:31)
"The blasphemy of the Spirit": The sin here spoken of is that blasphemy, by which the Pharisees attributed the miracles of Christ, wrought by the Spirit of God, to Beelzebub the prince of devils. Now this kind of sin is usually accompanied with so much obstinacy, and such wilful opposing the Spirit of God, and the known truth, that men who are guilty of it, are seldom or never converted: and therefore are never forgiven, because they will not repent. Otherwise there is no sin, which God cannot or will not forgive to such as sincerely repent, and have recourse to the keys of the church.
(Since you have sincerely repented, you should not worry about having committed an unforgivable sin. God will not turn away someone who is truly repentant.)