Going insane.
You may well not notice it. Why would you, your brain thinks it is normal. Maybe it starts small, like you get hung up on something weird, or start to misuse words. Then it gets worse, you are no longer making sense to others.
But you don't understand that. You know perfectly well your windows are missing and maybe the milk misplaced it and damn it why won't that purse shut up and help me look for it?!
Before long your impulse control erodes. Your sense of what is right, wrong, or even real is warping, vanishing as each day passes. Your start to see your partner, your child, that random guy at the store as a threat and you no longer have the part of you to restrain yourself. Eventually you find yourself in a care home, if you are lucky, or on the street if you aren't. You don't know why you are there, maybe.
But that isn't the most terrifying thing.
Imagine all I said, and now imagine that somewhere deep inside, there is still some part of you that is aware of it.
And you can't do a god damned thing to stop yourself.