Your grandfather died in a way that was peaceful for him but traumatic for you. I’m sure everyone has reassured you that in no way is it your fault that the call didn’t go through. You and your uncle tried your best but, as you say, it would not have made a difference if the medics had gotten there more quickly.
You are going through what is known in philosophy as an existential crisis. This is an absolutely normal response to a difficult death. Your grandfather’s passing made the finality of death real for you. Up until then, it was an abstraction. Yes, philosophically, we all know that everyone dies. But it is the first meaningful death we experience that makes that idea starkly real. That event makes many people question the meaning of life and the meaning of their own existence. It falls on us, then, to come to grips with the transient nature of life and to figure out how we want to live and love. These are the big, important questions. The fact that you are struggling with them tells me you are a sensitive and thoughtful person.
Modern life doesn’t give most of us the time we need to grieve and think and think some more — even though it would be helpful to do just that. If you can’t take time out from your responsibilities, what you can do is compartmentalize your feelings so you can function.
Here’s what compartmentalization means: Decide on an hour or so a day when you will give yourself permission to grieve your grandfather and to think about those big questions. Whenever feelings and thoughts come up for you at other times, remind yourself that you will deal with them during the special allotted time. Then make sure you do it. Your thoughts are less likely to intrude on your day (or your nights) if you give yourself a specified and regular time to store your memories and to think hard about the meaning of your experience.
I wish you well.