I really respect you for being so devoted to your Mom. It's got to be really hard to help, but at the same time feel helpless. I'm not great at it, but prayer helps me. There are respite programs that have someone come to your home and sit with/care for your Mom and you can go and run errands and basically, take some time for yourself. Are there any other family members or friends that can help out? You need to take care of yourself, too, otherwise you're not as good for your Mom. Do you have someone to talk to about your feelings and your life? When your Mom has good days, I bet she would appreciate just talking, or if you share some of what's going on with you, be it life in general or your feelings and concerns for her. Are you able to talk about that at all?
I'm very close to my Mom, too. I'm the one with cancer and at first, we didn't talk much about it, but now we are able to talk about anything again. There are tears, but sharing them with Mom instead of shedding them alone helps me. I wish you and your Mom the best. Many hugs your way!