My mother was diagnosed with uterine cancer 4 months ago
What have you been doing to cope?
My mood is relatively stable, but I'm worried about her.
I am sorry to hear that.  You are doing a nice thing by supporting your mom as much as possible during this hard time.
So I am a good son if I am sad about it?
It is good that you are supporting your mom. Just because you are sad does not mean you are providing any less support then someone who was not sad. In a way, showing your emotions could even motivate your mother to want to get better herself!