Research conducted at UC Berkeley showed that the more difficulty you have saying no, the more likely you are to experience stress, burnout, and even depression. Mentally strong people know that saying no is healthy, and they have the self-esteem and foresight to make their nos clear. When it’s time to say no, mentally strong people avoid phrases such as “I don’t think I can” or “I’m not certain.” They say no with confidence because they know that saying no to a new commitment honors their existing commitments and gives them the opportunity to successfully fulfill them. The mentally strong people also know how to exert self-control by saying no to themselves. They delay gratification and avoid impulsive action that causes harm.