The real question is this: Do we have the power to change our lives by ourselves - or do we need God's help? If we don't need God, then positive thoughts alone might work. But if we do need God's help, then positive thoughts by themselves won't change us into the people He wants us to be.
And we do need God's help, because in ourselves we don't have the moral and spiritual strength we need to live the way we should. Yes, sometimes we can change our lives to some extent, if we try hard enough. But God doesn't just want us to get rid of a few bad habits. He wants to take away everything bad and replace it with everything good - things like joy and peace and love and patience. He also wants us to come to know Him in a personal way, and some day go to be with Him in heaven forever.
This is why we need Christ, for only He can save us from our sins and give us a new heart - a heart that will love God and love others also. God's promise is clear: "I will give you a new heart and put a new spirit in you" (Ezekiel 36:26). This happens as we commit our lives to Christ and allow His Holy Spirit to begin changing us from within.
By a simple prayer of faith ask Christ to come into your life today, to forgive you and cleanse you of all your sins. Then ask Him to help you follow Him and live solely for His glory.