The difference between the scenarios you present is voluntary action. I'm absolutely not proposing that you should pop off and go for a Burton, and actively try to leave your nearest and dearest in the lurch - rather that people are resilient, and life goes on, and that we must believe in those we are closest to. I have life insurance to at least ensure the financial safety of my family should I cease to be.
I'm sorry for your experiences - most of my loved ones have met violent ends or have died of mercifully swift illnesses. Some have lingered, painfully, and wished for but could not have euthanasia.
Perhaps there's an argument for voluntary euthanasia being legal purely in the idea that it comforts the living with the knowledge that they need never suffer a slow death in the future.
If you just want to say that your loved ones will survive, that life goes on without you, and you shouldn't be overly concerned about how the world will get on after you die, that's reasonable. But I completely and utterly disagree with going from that to "Fearing death is utterly irrational." Fearing death because of the negative consequences it will have on people you care about is completely rational.
I mean, what's the difference between "It's OK if I die, my family will survive" and "It's OK if I run away with another woman and stop supporting my children, my family will survive"?
I also doubt your assertion that "more often than not" death comes quickly. Of the friends and loved ones I've had die, the vast majority went through a long decline first. In fact, I can't think of anyone I know who went quickly.