Don't have my Bible right here with me, but the passages are in chapters six and ten, I believe. One says that if you fall away, it is impossible to renew you to repentance and that you only have a fearful expectation of judgment to look forward to.
And the other says that if we sin after knowing the truth, there is no further sacrifice available.
Actually, UR is a historical teaching. Our worst critics that won't allow that this was taught in the Apostolic age will usually allow for it's beginnings in the early 3rd century, late 2nd. You've just heard one side of "historical teaching."
Impossible to renew to repentance before punishment/judgment. What's hard to understand about that, without imposing endless damnation on it? Christians fail to repent all of the time of all of the animal products and sweets being carried to an excess and come down with diabetes and other issues. Many people are punished/judged all of the time because they couldn't be brought to repentance before some evil befell them. Proves nothing to the point of endless punishment. I think that Hebrews 6 mentions age-lasting judgment, but of course the "Authorized Version" will insist upon "eternal judgment." Hebrews 6 goes on to talk about land that gets burned, which may have been talking about the impending judgment upon Jerusalem in 70AD since some scholars believe that Hebrews was written to the Christians in Jerusalem. Impossible to renew them again to repentance is a part of the Hebrews 6 passage, though, and I don't want to seem to evade. They've crucified to themselves the Son of God afresh and have held Him up to open scorn. The passage does seem to be peculiarly 1st century with the whole issue over whether Jesus really was the Messiah, and whether or not the Torah should be observed exclusively without regard for the teachings of Jesus.
Hebrews 6 ends with the promise of the unchangeableness of His counsel which He spoke to Abraham that those who blessed Him would be blessed and those who cursed Him would be cursed, and in his Seed all of the families of the earth would be blessed, or in other words, every cursing would have to end in blessing when it had acheived it's purpose. You really do need to restudy the whole Old Testament along the lines of judgment, sin being visited to the 3rd and 4th generation which is the Old Testament equivalent of not being forgiven in this age or in the age to come. In Judges the tribe of Benjamin got "cut off," yet the passage clearly indicates that all they meant was that there were no women for the tribe of Benjamin to marry. Jacob have I loved, but Esau have I hated. The descendants of Esau were absorbed into the house of Jacob in the time of the Macabees and no longer exist as a distinct people any more in fulfillment of His Word to Obadiah, and Romans 9-11 is saying that the world and the pruned branches will likewise and in like manner be absorbed into the New Man. All of this is so interrelated to where it's hard to answer something on an individual passage without seeming flippant or evasive if you're not on the same page with us about beaucoup Scriptures. You can't show me a single translation where the Valley of Hinnom is translated as "hell" in the Old Testament, unless it's in some odd paraphrase that I've never read through like the Message paraphrase or something like that.
Hebrews 10: if there's no more a sacrifice for sin REJOICE!! As far as how much sorer punishment do you suppose will they be thought worthy who've trodden under foot the covenant, which is probably more along the lines of what you were referring to. This word "punishment" in Hebrews 10 is a kin to a word used in the book of Acts that simply means persecution. Persecuted by the Spirit that they've outraged? Or simply outside of the realm of God's protection and turned over to the Judaizers (which is the point of the book of Hebrews in presenting an antidote to) for the Judaizers to do with one whatever they'd wish? Difficult to dogmatize about what's being spoken of. And then the preterists will be along shortly that'll muddy the water for you a little further and say that Christians that committed this sin were probably a part of the destruction of Jerusalem in 70AD when everybody got killed or sold into slavery, although there are historical references that'll claim that no Christians died in Jerusalem when it hit the fan in 70AD.
If we sin, there's no more sacrifice for sin because if we confess our sin, He's faithful and just to forgive us our sin and to cleanse us from all unrighteousness. If these answers don't suffice, press me and the rest of us with more specific questions about the passage and between all of us, I'm sure you'll come away with a BIBLICAL ANSWER that'll suffice.