Mental discipline means taking control of wandering thoughts that lead to anxiety or unhealthy comparisons with other believers. Instead of comparing yourself to mega-pastors or seemingly "perfect" Christians, remember that you're already complete in Christ through salvation, the Holy Spirit's presence, and God's grace and mercy.
We're not all called to the same ministry, but we are all called to Jesus. We're one body united in one spirit, but with different parts and gifts. The key is disciplining your mind on the things of God through His Word and exercising self-control in every area of life.