Does anyone else worry about pets dying?

I have 2 cats who are 6. The past 2 days I've been all emotional thinking about the day they pass. Even looked on YouTube at cats being euthanized to get an idea of what it involves. My pets are family and have no idea how I will cope when they go. Specially when 1 goes and then the other is lonely and the knowing I'll have to go through all the emotion again. They are both healthy as far as i know, go to the vets every 6 months and have them insured. How have others coped with a pet dying? Trying to enjoy now and stop worrying!