Let's look at it in the context of the whole book of Hebrews. There is a few parts that belong to this same topic, for example Hebrews 6:4-6
"
It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen 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"
and Hebrews 3:14
"
We have come to share in Christ, if indeed we hold our original conviction firmly to the very end".
The part you quoted as well as Hebrews 6:4-6 say that we can receive the Holy Spirit and be forgiven our sins, and then fall away still, return to our old, sinful lifestyle and "crucify the Son of God all over again". You can accept Jesus in your life and one day turn your back on Him and live life your own way as if you never knew Jesus.
But the Bible says in many occasions that the chosen ones, the people who are saved, can never lose their salvation, nothing can snap them out of God's hand! Is this a contradiction?
It is not a contradiction. Look at Hebrews 3:14: it says "
if" we hold... firmly to the very end,
only then we have come to share in Christ. In other words: Once you are saved, you are always saved. But whether you had been saved is only the case
if you stay firm to the very end, until Jesus comes back. Hebrews 6:4-6 says you can receive the Holy Spirit yet not be saved. Only if you endure to the end you had been saved the day you accepted Jesus in your life.
Should you worry now? Could it be that you are not saved? Well, as long as you worry about it you have not fallen away

All of us have times closer to God and times further away from God, times in light and times in darkness. When you sin as a Christian it doesn't mean that you are not saved, you can always repent and turn back. Hebrews talks about those people who turn
away from God on purpose and make no effort to repent anymore, who don't
want to turn back to God.