Fierce Mercy

I haven't read any of Brennan Manning's books, but he has a book that is out in July called The Wisdom of Tenderness, How God's Fierce Mercy will transform you. I don't know about you, but I'm not sure I really understand mercy. I mean, yeah, I know the word and what it means, but I don't grasp and live in mercy. As I was driving to work this morning, I thought about God's forgiveness and how I just keep it to a prayer for "I'm sorry God. I'll never do this again." But is that what God wants us to get out of forgiveness? And what about that wording "fierce mercy?" Meditate on mercy and enlighten me friends.