I know how you feel. I have also lost multiple family members in a very short time. It does get better, even though right now it feels like it never will. It sounds like it would be a good idea to talk to a grief counsellor. They will be able to advise and support you throughout this time. Religious leaders like vicars and priests etc can provide excellent support as well. Don't worry about bothering them, it's part of their job to support people in their times of distress.
Is your Grandma in any form of hospice care? Usually, this would be as an outpatient, but there are inpatient hospices as well. A hospice can provide excellent care and support, both for your Grandma and your entire family. It can sound contradictory, but they are very happy and joyful places to be.
Make the most of the time you have left with your Grandma. Spend as much time as possible with her and say everything you want to say to her.
What you are going through is a natural part of grief. It is normal. It will get better, eventually. There is no timetable for grief.
If you ever need to talk to someone completely non judgemental, there are grief helplines that you can call.
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