The doctrine of verbal plenary inspiration of Scripture means that every single word of the Bible is "breathed out" by God (2 Timothy 3:16). So when Paul says "I, not the Lord" in 1 Corinthians 7:12, he is clarifying that Jesus gave no direct command regarding the specific situation of a believer having an unbelieving wife. Paul demonstrates godly wisdom applied to the issues about which the Corinthians wrote as he responds to their questions related to marriage (1 Corinthians 7:1, 6, 40).
Paul is not giving human advice; rather, he wrote these words under the inspiration of the Holy Spirit to a specific group of people at a specific time. Thus, they are personal, but they are relevant and instructive to us today in the same way all Scripture is (2 Timothy 3:16–17).
In 2 Peter 3:15-16, Peter notes three important aspects of Paul's letters. First, he states some parts were hard to understand. Second, Peter notes that some had twisted Paul's words "to their own destruction." Third, Peter equates Paul's writings with the "other Scriptures." These other Scriptures were the Old Testament writings, works referred to as inspired by God by the apostle Paul (2 Timothy 3:16-17), the apostle Peter (2 Peter 1:20-21), as well as Jesus (Matthew 5:17-20). Peter here makes the clearest comment in the New Testament that Paul's writings are inspired. This was not only the view of Peter, but was the accepted view of the earliest church fathers who included Paul's letters along with the other New Testament writings as inspired Scripture.
Though some people have misinterpreted Paul’s comment in 1 Corinthians 7:12, we can be confident that it, like the rest of Paul’s writings, is inspired by the Holy Spirit.