if we sin willfully after we have (hebrews 10:26?

NO more sacrifices for sin means that Jesus, by dying on the cross, ended the need of any more sacrifices and he became the New Covenant. There was no more the need for animal sacrifices required under the Law (the old covenant).

Jesus said on the the cross, "it is finished", meaning that the old covenant was replaced proved by the tearing of the curtain in the Holy of Holies.

If we truly repent of our sins and ask Jesus to be our Lord and Savior, even when we sin we still belong to Jesus. We can't lose our salvation because no one can steal us from the grasp of the hand of Jesus, no one. The Apostle Paul said "being confident of this, that he who began a good work in you will carry it on to completion until the day of Christ Jesus" Philippians 1:6

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For if we sin wilfully - If we deliberately, for fear of persecution or from any other motive, renounce the profession of the Gospel and the Author of that Gospel, after having received the knowledge of the truth so as to be convinced that Jesus is the promised Messiah, and that he had sprinkled our hearts from an evil conscience; for such there remaineth no sacrifice for sins; for as the Jewish sacrifices are abolished, as appears by the declaration of God himself in the fortieth Psalm, and Jesus being now the only sacrifice which God will accept, those who reject him have none other; therefore their case must be utterly without remedy. This is the meaning of the apostle, and the case is that of a deliberate apostate - one who has utterly rejected Jesus Christ and his atonement, and renounced the whole Gospel system. It has nothing to do with backsliders in our common use of that term. A man may be overtaken in a fault, or he may deliberately go into sin, and yet neither renounce the Gospel, nor deny the Lord that bought him. His case is dreary and dangerous, but it is not hopeless; no case is hopeless but that of the deliberate apostate, who rejects the whole Gospel system, after having been saved by grace, or convinced of the truth of the Gospel. To him there remaineth no more sacrifice for sin; for there was but the One, Jesus, and this he has utterly rejected.

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The knowledge of truth means the message of Christ dying for us as the Ultimate Sacrifice and believing on Him as the Son of God as the way to get to Heaven. Knowing this, we should do our best to turn from sin because, the more we sin, the more accustomed we become to it and the more we forget about what Christ did for us. This is why the verse says that when one keeps sinning after knowing the truth that "only a fearful expectation of judgment" remains. Therefore, it takes more than believing in Jesus to get into Heaven, but it also takes remorse for our sins and a fear of God because we know Hell awaits if we are not remorseful.