Hmm, I've given this some thought off and on over the years. I have a fear of spiders, small, tight spaces, the kinds of things most people fear. As for the one thing that terrifies me, I'd venture to say dementia. Losing control of my faculties and not knowing who I am, where I'm at, or who's looking after me or my things is terrifying. With the exception of a few half cousins, I have absolutely no family. It terrifies me as to what will become of me if I develop some form of dementia. My mother took all my possessions once many years ago, so I have a constant fear about that happening again. That's probably why my house is like a fortress. I've taken measures (legal documents) to ensure my things are secured and a "trusted friend" can make decisions for me, but it still terrifies me all the same.