How is it possible that I don't want to die?

I'm lonely by cirstance and I'm isolated by choice, my kids are grown and have gone on with their lives and no longer need me. I have no interests, and no real pions any more...and yet I still don't have the feeling of wishing for my own demise, and actually have a rabid fear of death. What is it that makes me trudge through day upon day of nothing and cling so desperately to a life that I don't want?