God’s mercy is new every morning. (Lamentations 3:22,23 KJV.) Each day we can find a fresh place to begin.
I like the way God has divided up the days and nights. It seems to me that no matter how difficult or challenging a specific day may be, the breaking of dawn brings new hope. God wants us to regularly put the past behind and find a place of "new beginnings."
Perhaps you have been trapped in some sin, and although you have repented, you still feel guilty. You may be assured that sincere repentance brings a fresh, new start. First John 1:9 promises a thorough cleansing if you will freely admit that you have sinned.
Once you understand the great mercy of God and begin receiving it, you are more inclined to give mercy. You may be hurting from an emotional wound. The way to put the past behind is to forgive the person who hurt you. Forgiveness is always involved in putting the past behind.
God has new things on the horizon of your life, but you will never see them if you live in and relive the past. Thinking and talking about the past keeps you trapped in it.
Every day is a new day. Don’t waste today by living in yesterday.