...I overthink about really negative things....

I'm so sorry to hear you are consumed by such fear. People live and people die. I like to look at it as "making way for the new babies to have their turn at life". Earth would be uninhabitable if us oldies didn't die off eventually! We can have an understanding of the cycle of life but that is separate to grief. Grief is something your friend will be feeling atm. You might not have been close enough to her father to miss him, but yes, it seems to have reawoken you to the reality of thefragility of life. We never know when or how we are going to die, but we are all guaranteed to experience it at some time. It is the natural order. No matter how much we understand that, it is still painful when someone we love dies. We are used to having them around and their death leaves a gap in our lives... the process of that "hole" healing over is what grief is about. Possibly you may be able to support your friend at this time. She would probably welcome someone to talk to about "what do you think happens to us when we die?". I wouldn't tell her about your fears because she has had a major trauma and she needs people to be there for her atm and not demand her comfort and support with their own stuff. I think you need to talk to your parent/s about your fears and describe to them how your fears are impacting on your life. I'd be really interested to hear their take on what you are experiencing.