Mrs.Whatsit was sad in the beginning of the chapter because she saw a star die while fighting the Black Thing. She was a star once and the exact same thing happened to her.
On pg. 103, Charles Wallace says, "I see. Now I understand. You were a star once, weren't you? "
Mrs Whatsit covered her face with her hands as though she were embarrassed, and nodded.
"And you did what the star just did?"
With her face still covered, Mrs Whatsit nodded again.
So, Mrs Whatsit sacrificed her stardom to fight the Black Thing. She was a hero, but she was still sad.
Write something about yourself. No need to be fancy, just an overview.