The cells of your body are constantly responding to your inner environment. Mitochondria are firing up in your muscle cells in response to movement. Your digestive tract is interacting with the foods you eat. Your nervous system is responding to the threats you perceive. Your endocrine system is reacting to the way you feel.
Neurotransmitters, the molecules of emotion, like serotonin are found in the brain, but also in the gut. How you feel affects your physiology. You can affect your physiology by changing how you feel through movement, different food choices, and how you think about things. Constant worry wears down your immune system. You can set aside a time to worry and then stop it. You can pay attention to the content of your thoughts and change them. Instead of worrying, you can picture yourself lying on a on a beautiful warm white sands beach, listening to the sounds of the gentle surf, slowing your breathing, relaxing. Your body will appreciate the change of scenery.