The "sin willfully" in Hebrews 10:26 is referring to the specific sin of apostasy which is set apart from all other willful sins that can be repented of.
In this video we will examine 1 Corinthians Chapter 5 to see an example of how someone within the church of Corinth who sinned willfully by fornicating with his father's wife could be restored through repentance. It helps us to see that salvation can be lost if we don't continue to live a repentant life, but it also helps us to see that we can repent of willful sin and be forgiven. Anyone who has believed that Hebrews 10:26 is referring to any general willful sin is mistaken. It is specifically referring to the willful sin of apostasy (Jewish Christians reverting to Judaism) when we examine the entire context. All other willful sins can be forgiven. So don't let Hebrews 10:26 steal the hope you can have in Jesus Christ. Obey the gospel (Acts 2:38) and be born again, living a repentant life empowered by the Holy Spirit.