This is where it pays to dig into the original languages, or at least to compare several translations, because the verb tenses can be so important. Here is my rendering of that verse:
For if we are sinning willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, -Heb 10:26
Notice that the verb
sin is in the present participle, indicating ongoing present action. The Disciple's Literal says the same thing a different way:
For while we are willfully sinning after the receiving of the knowledge of the truth, a sacrifice no longer remains for sins,
And very accurate mainstream versions such as ESV and NASB concur by rendering it "if we go on sinning..." NIV, LEX, NET say the same thing in slightly different ways.
The accurate sense of the verse is that while you are in unrepentant sin you are in big trouble. It is not saying that once you sin you cannot repent, confess, and return.
BTW, the same thing is true of the verse about looking back from the plow causing one to be unfit for the Kingdom. It's not a "look back once and you're fried" situation, like Lot's wife. Rather, it's "you can't plow straight rows unless you're looking ahead in the direction in which you're going".
Whatever your sin was, you obviously regret it now. Make your repentance formal by confessing it to the Lord. Do it with faith in His goodness and kindness, that He will graciously receive you back, like the father of the prodigal son at Luke 15. Remember, this same author of Hebrews only a few verses later describes what is needed in order to please God:
And without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who seek Him. -Heb 11.6
