You are seeing things merely from a human point of view, not from God's - Mark 8:33
Peter must have felt great when Jesus publicly blessed him from understanding Jesus' true identity
Then it was shown that he needed to be knocked down a peg
Jesus stunned his disciples about him going to Jersulem and Peter scolded Jesus because of it
Peter was out of line and Jesus would not allow him or any other disciple to act as roadblocks to the path God set for him
Jesus knew Peter had failed to see things from an eternal perspective
Jesus was the conquering Kig but he was also the suffering Servant
Peter was to play a big part in Jesus' mission but he first needed to humbly trust in the Lord's plan