Bitterness comes from our own personal reaction to irritations and problems in life. We must recognize that people or illnesses don't make us bitter or resentful regardless of what they do to us. It is our problem because we have wrongfully responded to the problems and irritations of life. Our resentments call forth certain hormones from the pituitary, adrenal, thyroid and other glands.
Excessive hormones can cause disease in any part of the body to complicate the healing process.
Major hindrances to spiritual development take place. It requires emotional energy to maintain resentment. The moment I start resenting a person, serious mental consequences occur. When I'm served steak and potatoes it might as well be bread and water. Bitterness can be healed by forgiveness.