While positive affirmations provide the words to create new beliefs, visualization provides the mental imagery to see yourself actually living these beliefs. Your mind then records the mental pictures as real and valid. You can see yourself speaking confidently and acting confidently. You can see yourself being a stronger person in your everyday life.
Your brain is divided into two parts: the left, logical side of the brain and the right, creative side. Most of our lives are spent using the left side, logically and rationally solving problems and performing tasks, using words and concepts. The right side sees the world in pictures and is associated with emotions. Visualization uses images and emotions to get training for actual performance.
Visualization is seeing bright, colorful, and vivid pictures of how you want to be. Your mind records these mental pictures as real and valid. Because your mind supports the images it has and wants the mental images on the inside and the physical images on the outside to be consistent, it will try to match your outer reality with your new mental reality.
As you say your affirmations and visualize actually being that person with healthier self-esteem, your mind records the image as happening in the present moment. It seems to your mind that you’re living it right now. When your vision is clear and certain, your mind is then alert to what you need to do to accomplish that.