I feel so mad.
What is causing you to be mad?
I didn't get chosen for the part.
People get frustrated all the time and it is understandable to be mad, but try to calm down and think about why you feel the way you do.
I'm not mad, I'm just disappointed
No matter what the reason is for why you're upset, just know that it is normal and natural. Try to understand how you are feeling, and try things like healthy food and exercise to see what helps you deal with the stress and makes you feel better.