I'm so sorry to hear that you're in this situation. But my only advice would be not to jump to conclusions yet. For all you know, it might not be cancer. Even if it is, it could be small and easily treatable. I understand why you're worried but I think the best thing to do right now is think positively - otherwise it will lead down an endless rabbit hole of doubt and worry.
Even if the worst does turn out to be true, just remember that there's nothing you personally can do about it except spend time with your mum and give her the support she needs. But also remember to take care of yourself. Talk to friends, siblings, family, counsellors, etc. or other people you can share your feelings with. Stay active, eat well, sleep well and make sure you exercise, as well as continuing with your hobbies and having enough time to relax. I'm sorry that I can't be of more help but I think looking after yourself properly is the best way you can battle through this.