From Paramahansa Yogananda (who wrote Autobiography of a Yogi):
"Negation acts like an insulator that inhibits the flow of God's power through you and thus causes inharmonies of body, mind, and soul. Criticism, anger, envy, suspicion, jealousy, fear, hate, doubt, laziness, worry, anxiety, thoughts of lack and so on are all forms of negation. Watch your thoughts. Stop them from pointless wandering, and make them obey you. Train your mind to think constructively at all times. A joyful, thankful attitude will carry you a long way toward the goal of bringing into your life only the good things that you desire."
"Change your thoughts if you wish to change your conditions. Since you alone are responsible for your thoughts, only you can change them."