By repentance in prayer, I mean taking the time before God to search your heart and repent of anything that has come between you and Him. Psalm 19:12-13 expresses it well,
Who can understand his errors? Cleanse me from secret faults. Keep back Your servant also from presumptuous sins; let them not have dominion over me. Then I shall be blameless, and I shall be innocent of great transgression.
Verse 12 begins with the question, "Who can understand his errors?" The psalmist is telling us, "You will not always know when you do something wrong. You will not always know when you get into an area that is not right."
What David is pointing to are the secret faults and presumptuous sins which can still have dominion over you—even though you may not be aware that what you did was wrong.
For example, sometimes we can allow attitudes to get into our hearts that we don't realize are inconsistent with God's character. Or sometimes we can do and say things that are detrimental, not only to us, but to others, and not really understand the damage we have done.
How do you deal with these sins? You come before God and say, "God, put the spotlight on anything in my life that has raised a barrier between You and me, and I will repent of it."
So when you pray, ask God to reveal any sin in your life you may be overlooking. God will honor your heart of repentance.