Forever Dark,
I am so sorry to hear about your mother. That is so hard. I'm going to be thinking about you and sending some calm, comforting energy your way.
I'm assuming you live near your mom. Are you a caregiver to your mother? Do you have other family in the area? As a person becomes sicker, more and more tasks and care falls to the family, and it adds even more stress, becomes even more busy and affects sleep, etc.
I found support is a key part of managing stress through this. If you do have family in the area try to lean on each other for support, and also spread the responsibilities amongst you. If you don't have family in the area and it's just you and your mom, see if you can line up some support now (to help drive her to appointments, deliver meals to her, etc.) There are a lot of social service agencies and nonprofits that offer services. If your mom has been assigned a social worker, they can help. Your doctor's office may have some ideas. If she is in (or will be in) hospice, this aspect of things gets a little easier. All her needs are cared for there.
Breaks also helped me. Time out, away from the situation, both mentally and physically. Do whatever you love to do most, but get at least one afternoon off a week.
Other than that, talk to and lean on your friends (that definitely includes us), and remember to eat, sleep, and breathe.