Hi there. My husband was diagonsed with stage 4 throat cancer in March and is just 6 months post-treatment. We came very close to it being too far to treat. I don't know how helpful this will be, but know that you are not alone. It's ok to feel bad, it is about you. You have to take care of yourself and be strong. Get another opinion, research your options, read posts about this type of cancer. This is a great place for information and support.
I lost a very dear friend in February after a 11 year battle with breast cancer. She died before Dave was diagnosed, but I remember her telling me that if she could just help one person deal with what she's had to endure, then it would all have been worth it. I think of her every day, and I know it's because of her that I was able to get our family through this difficult year.
Enjoy the time that you have with your mother. Try to do some 'normal' things with her - by 'normal' I mean things you used to do together before she got sick. Right now she probably feels like her life is over, that she's a burden to her family and friends. Try to give her even 5-10 minutes a day of something entirely different than cancer.
Best of luck to you, you're going to be ok.