At the heart of the spiritual life is self knowledge. To truly "know" one's self is the bedrock of our knowledge of God. We can spend an inordinate amount of time and energy "projecting" dark, inner realities in other people because we don't want to "see" those realities within our own hearts. It's the easy way of not facing the unpleasant truths about what really lurks deep within our hearts. We project these truths into others so that we don't have to deal with them in our own lives.
Jesus brilliantly says it with clarity: why do you keep noticing the little splinter in your friend's eye but fail to perceive the "wooden plank" in your own? We love to notice the little splints, the faults and sins of other people. The faults we "see" in others find their roots right within our own hearts.
We must look within our own hearts as we look in a mirror. We have to have the courage to truly "see" the darkness within: the hurts, the painful memories we'd rather bury, the guilt over past actions. When we finally "see" these awful truths, we are free of them. As we look in the mirror honestly, we finally "see" the face of Christ. In our own face.
Peace, Fr. Frank