This is a hard one. On the one hand I think, well no kids, no one you owe anything to and it is your life so of course YANBU. On the other hand if someone is so central to your very existence then this isn't healthy. By our very nature, we are temporary and our mental equilibrium ought to reflect this, when it doesn't, something is off-kilter. If I were you I would want to search and find meaning beyond that one person.
When I was in high school a friend of mine lost his mother to cancer (we were about 12 at the time), within a couple of months his father had killed himself. Now, apart from the fact that he left a teenage son all alone and committed one of the most selfish acts I'd ever seen, I remember not understanding how the man's mind was working to not see beyond both himself and his wife. To put it into perspective, I had lost my brother a few months previously and seen the grief of my parents and grieving myself and that pain is pretty indescribable. But I also saw that the world turned and somehow my parents had tojust deal. It was terrible.
Death is a hard thing, the hardest. But our relationships need to take the inevitable end into account. It is a short life we have, let's make it a good one. I don't think your husband would want you to die after his passing. If he loves you, which I am sure he does, he would want you to live and experience and enjoy the time you have left.