It is quite natural to be affected by the fact that your grandmother is ill and it is normal to be sad.
If you have someone who is close to you, you should speak to them, sharing your feelings may help you.
If the feelings of depression
become overwhelming and/or you develop thoughts of self harm you should speak to someone close to you and a doctor as soon as you can.
If your grandmother is able, try to spend some time with her, talk and read to her, let her know that you are there. It may be difficult to see her looking ill but if you don't get to spend this time with her , it may make you feel worse later and it will also help you with closure.
Remember the times you would have spent with your grandmother when she was well and embrace these times.
Unfortunately death is part of the natural part of life and happens to each and every one of us. Your grandmother had the oportunity to participate in your life and no doubt ,even when she is gone, what she has taught you will live on in you.
Let your life be a reflection of her