Thoughts don't change anything unless they feel true. If you state a thought that you don't believe, you won't sense/feel it in your body and there will be no energetic charge. It will fall off you like water off a duck's back.
If you say “I'm a horrible person” and that thought/belief feels true, and you are muscle tested, the muscle being tested will remain strong, meaning your body is saying Yes to this belief, it senses and feels this as a truth. If you say “My name is Ann” when it is in fact Sarah, this won't feel true for you and your muscle will test weak.
The thought or belief would have no validity or influence without the sensation and feeling in the body that it is true.
What gives rise to a thought/belief such as “I am a horrible person”? Life events. Events in which we felt bad about our self. Initially this belief could have started out very early in life, as physical sensations, it might have felt like a sharp jab in our stomach. If the sensation becomes too overwhelming, we won't/can't feel it which creates a short circuit in our energy system. The sensation and later, the attached emotional charge become stuck. To make some sort of sense out of what is happening and to feel in some way safe and in control, we'll arrive at conclusions. If we need the person who is hurting us, we can't hold them responsible, so we'll come to the conclusion that we are to blame. There must be something wrong with us, for this to be happening, otherwise it does not make sense. Hence the belief “I'm a horrible person”.
Thoughts and beliefs affect and shape us when we sense and feel they are true. Positive affirmations won't work if they don't feel true. We need to address the underlying sensations, emotions and feelings that give rise to these thoughts and beliefs, not the other way around.