I have no problem with accepting that Paul wrote the book of Hebrews. It has historically been believed that he was the author. But, regardless of that, since it IS Scripture, why do you not think it is applicable to us today? It definitely is geared towards the Jewish mindset, but that does not mean we cannot trust in the doctrines put forth.
It's not that I have a particular problem that Paul wrote Hebrews, or not. It's that Paul was given the Dispensation of the grace of God, the One New Man, where there is no distinction between Jew and Gentile. We are all on equal standing before God during this present dispensation. Yet Hebrews is written to the Hebrews. The middle wall of partition that Paul spoke of is back. Which means the dispensation of the grace of God is over and the dispensation of the Millenial Kingdom has begun. With Hebrews through Revelation as God's personal word to those saints going through the tribulation and enduring to the end. He has started dealing with Israel as a Nation again. And He has, through the Book of Hebrews, shown those who are going through the trib. that Christ is better than the Prophets (Jewish), Angels, Moses (Jewish), Joshua (Jewish), Earthly Priests (Jewish) and all Old Testament Saints (Jewish), the MMelchisedec Priesthood supercedes the Aaronic (Jewish) and the New Testament better than the Old Testament (Jewish). It is about God dealing once again with Israel as a Nation of priests and their future blessings to all nations.
It is not a continuation of Paul's commission to Jew and Gentile, forming the Church the Body of Christ, who, at this point in God's timeline, has been raptured.