SAD is not an emotion. It is Seasonal Affective Disorder (SAD). SAD is the colloquial term for an official diagnosis of Major Depressive Disorder with a Seasonal Pattern.
Self-Compassion Is Something We Can All Do
Practicing Behavior Activation
Ways to Stop Doing Depression
Two Sources of Your Vulnerability to Depression
Feelings, Thoughts, Actions: How Depression Develops
You can say two things about the emotional upheaval that happens with loss, rejection, or disappointment. First, these emotions are normal. Second, these emotions are not only normal, but they are also important. These feelings tell you that you are experiencing distress in some area of your life that really matters.