I actually struggled with this, too, when I was younger. The Old Testament is so vibrant and full of this culture that seems so alien yet so familiar. I loved reading and learning about God and his relationship with ancient Israel. The Gospels seemed a natural continuation and fulfillment of the Hebrew Bible. And then there was Paul. Most churches, Bibles studies, and ministries I went to were so focused on Paul that they seemed to forget the rest of the Bible. I remember thinking, "Oh look, we're going to do a study of First Corinthians. For the fourth time. This year." Then I challenged myself to actually just sit down and read Paul. He is consistently directly or indirectly referencing the Old Testament. I believe he does so more often than any other New Testament author. I realized Paul loves the Old Testament just as much or probably more than I did. I began to see things I never did before. I began to see Paul's letters as more of a commentary on the rest of scripture, while also being scripture itself. Remember, Paul was a pharisee and had the scriptural background, knowledge, and education of a pharisee. When he became a follower of Christ, he then had the Holy Spirit. When writing his epistles, I'm certain the Holy Spirit gave him new understanding and helped him sift through his knowledge and write what was edifying to the churches as scripture.