The Squid can understand your fears. We have dealt with three deaths that hit close to home in our lifetime. Each time we deal with this, We end up laying in our bed, awake, wondering if our mother will be next. What will we do when we lose her?
We think this is a natural response. Dealing with the death of a loved one opens up questions we don't normally think about. What we can suggest, is that next time you start feeling that way, turn your thoughts to how lucky you are to know someone who means that much to you. We will all die in the end, and there is no way of knowing how or when. Just spend time with that person. Enjoy his/her company! You may have another 30-50 years with them!
Keep in mind too, that when someone dies, they don't disappear off of the face of the earth. We continue to tell their stories, remember those funny moments, and talk about the cool things they did.
You aren't alone, It's just part of the grieving process, you will just have to work through it. Time heals everything.
We are sorry for your recent losses, and we can assure you, it will get better.