The passage is not talking about regular sins but in Matthew Henry's words it is talking about a total and final falling away.
Matthew Henry’s Concise Commentary
The sin here mentioned is a total and final falling away, when men, with a full and fixed will and resolution, despise and reject Christ, the only Saviour; despise and resist the Spirit, the only Sanctifier; and despise and renounce the gospel, the only way of salvation, and the words of eternal life. Source
John Gill's Exposition of the Bible
For if we sin wilfully
Which is not to be understood of a single act of sin, but rather of a course of sinning; nor of sins of infirmity through temptation, or even of grosser acts of sin, but of voluntary ones; and not of all voluntary ones, or in which the will is engaged and concerned, but of such which are done on set purpose, resolutely and obstinately; and not of immoral practices, but of corrupt principles, and acting according to them; it intends a total apostasy from the truth, against light and evidence, joined with obstinacy. Source
Hebrews 6:4-6 (New International Version)
4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the powers of the coming age 6 and who have fallen[a] away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Hebrews 6:6 Or age, 6 if they fall Source
2 Peter 2:20-21 (New International Version)
20 If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and are overcome, they are worse off at the end than they were at the beginning. 21 It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. Source
Reasons why the willful sinning is to be understood as final falling away
1 John 2:1 NIV
My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One. Source
One of the things Jesus is doing is that He is an Advocate mediating on our behalf. David for example sinned against God and repented.
Revelation 2:5 (New International Version)
5 Consider how far you have fallen! Repent and do the things you did at first. If you do not repent, I will come to you and remove your lampstand from its place. Source
Regular sins willfully done can be covered by the death of Jesus through repentance and confession. However willful sinning as in a final falling away cannot be covered.