You are mistaken. The apostle Peter -- a very strict Jew -- placed ALL OF PAUL'S EPISTLES alongside the Scriptures (the Tanakh).
And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other Scriptures, unto their own destruction. (2 Peter 3:15,16)
Which meant that Paul's personal opinion had no bearing on Scripture truth. If he used "I" it was with apostolic authority and under divine inspiration. What he meant is that he was speaking as God had directed him. Therefore his recorded words are deemed to be the Word of God.
For who hath known the mind of the Lord, that he may instruct him? But we have the mind of Christ. (1 Cor 2:16)
For this cause also thank we God without ceasing, because, when ye received the Word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the Word of God, which effectually worketh also in you that believe. (1 Thess 2:13)
All of Paul's epistles are a part of the New Testament, and genuine Christians have never suggested at any time that some Scriptures are merely Paul's personal opinions, to be rejected as some deem fit!