You have my deepest sympathy! It sounds like he was a wonderful grandfather and husband.
Part of the flight instructions is a reminder to put your oxygen mask on first, before you begin assisting others. It makes sense. You can't save someone else if you can't breathe. I appreciate that you want to be strong for your grandmother, but in order to be there for her, you need to be there for yourself first. If you're not able to get out and about due to your caregiving duties, maybe an online grief support group would help. You can find a list of groups at https://www.verywellmind.com/best-online-grief-support-groups-4842333.