If you are serious about reforming your prayer life, you must begin with your heart. Unconfessed sin, nurtured sin, will always be a barrier between God and those he has made in his image.
True, sometimes when we try to clean up relationships that have soured in the past, the other party remains intransigent. But that is between that party and God; you and I must watch our hearts.
This is true even when the offense has been entirely on the other side.
-- p. 76, A Call to Spiritual Reformation by D. A. Carson