“Prayer for peace in my heart” is partially based on Ephesians 2:13-18 which speaks about how Jesus is our only true peace. He makes peace between you and God by ending all conflict and hostility. But beyond just an end to disagreement, true peace is a unified heart. Pray for peace in your heart and God will give it to you.
Be certain about anything. When you receive God's word, you can be sure the blessings will follow. This prayer for discernment is an example of the process... go to God for anything, and he will give you clarity!