If you read through his epistles, Paul first hustled himself to the 12 in order to be accepted and constantly complained that they didn’t accept him as equals to them. If Paul was so sure, why would he complain so much?
Paul was sent forth as another apostle, to bring another message to a world of lost sinners: "the gospel of the grace of God." That is what the book of Acts is, a transition period from law to grace, from a kingdom of believers to a body of believers, and from the nation Israel to the oneness of Jews and Gentiles in one new man, the Church the Body of Christ.
The 12 disciples were still in Jerusalem, doing exactly what they were told to do by Christ, preach the kingdom gospel to Israel first and then the rest of the nations. But they weren't getting far, Israel wasn't accepting it.
You can understand the differences between the 12 and Paul if you understand that their calling and their messages were different. One was to A NATION. And one was to THE NATIONS. One concerned prophecy. And one concerned a mystery.