I agree with both the posts above - if you are in a lot of pain nobody would blame you for putting yourself first. And if you talk to your mum extensively about your problems, and don't hold back, maybe it would help her understand your need to ctb. Maybe think about writing a long letter spelling out in black and white how terrible your life is, and I'm saying about writing a letter as opposed to just talking because if you write it all down your mum can refer back to it and it would help her understand. Im sorry to hear that your sister lives far away, it means you won't have the peace of mind of knowing she could take care of your mum if you weren't around any more.