Try to take in the reality of death. Anyone's, no one's. Our loved ones will die. We will die. Our pets will die. Everything dies at some point in time. Maybe it's this reality that you are struggling with.
Think how badly our world would be if everyone lived forever. Dying is not within our control. But living now is within our control. The quality of your life and the quality of your relationship with your parents is within your control. Try to focus on making the best of times with them be now, so when their time comes, they will leave with happiness in their hearts and not sadness. Your life beyond theirs will be relatively free of heartache if you make today count. Good luck!