Answer: It should first be noted that any sin which is sincerely repented of is never unpardonable (unforgivable)! God promises that through the blood of Christ, meaning his sacrifice, we can be forgiven of all disobedience and reconciled to him (Matthew 12:31 - 32, John 3:16, 1John 2:1 - 2, 3:5, Romans 5:9 - 10). Your question on the unpardonable sin is closely related to the idea of what does it mean to blaspheme God.
It is interesting to note that the Old Testament broadly classified sin into two categories, one of which only roughly approximates disobedience that is unpardonable. The first and largest category of disobedience were those committed out of ignorance and weakness. The second category are those committed 'with a high hand,' meaning those in direct defiance to God.
An example of disobedience done 'with a high hand' (a rough Old Testament approximation of the unpardonable sin) is the man found collecting wood on the Sabbath in clear defiance to what he was told (Numbers 15:32 - 36). The two broad categories of disobedience discussed in the New Testament are those that can be forgiven and those that cannot (the unpardonable sin).
The most controversial verses related to your "unpardonable" question are in the gospels. Jesus taught about a special class of disobedience in his scathing rebuttal to some religious leaders. They accused him of performing miracles and casting devils out through and by the power of Satan rather than by the power of God. Christ warned them that blasphemy against God's spirit would not be forgiven.
. . . every sin and blasphemy shall be forgiven to men except the blasphemy against the Holy Spirit; that shall not be forgiven to men . . . but whoever speaks against the Holy Spirit, it shall not be forgiven him . . . (Matthew 12:31 - 32, HBFV throughout, see also Mark 3:22 - 29).
What makes the trangression Jesus discussed in Matthew 12 unpardonable (unforgiveable)? It is a deliberate renunciation of God. It is a knowing and willful rejection, without sorrow or any regret, of Jesus' sacrifice and the gift of salvation. It is the ultimate choice to consistently disobey God and forever refuse to repent. Satan the devil has not deceived those who have committed this kind of transgression. Those who commit this sin have seared their consciences as if with a hot iron (1Timothy 4:2).
We also find warnings related to sin that is unpardonable in the book of Hebrews. It states that repentance is impossible for those who fully understood God's truth but later rejected it completely (Hebrews 6:4 - 8). It also warns about those who fully reject God with the following.
. . . there is no longer any sacrifice for sins (we have crossed the line and have arrived at a state of mind that is unpardonable), but a terrifying (frightening, horrifying) expectation of inevitable judgment and of fierce fire (the lake of fire), which will devour the adversaries of God . . . (Hebrews 10:26 - 27, see also verse 29).
The unpardonable sin is not done by accident. It is not a single act per se, but rather the outcome of a process of knowingly rejecting righteousness for a period determined by God. It is not done through the same kind of temptation, deception and weakness that ALL humans are susceptible to (1Corinthians 10:13).
In the final analysis, anyone who is worried that they might have committed an unpardonable sin almost certainly has not done so. Those who do disobey in such a fashion not only understand what they are doing but also, like the devil and his demons, do not worry about it.