Why not curse your main character or villain with the inability to let a thought go? Why not riddle them with depression or rage? Why not make the darkest depths of the night impossible to sleep through and haunting?
I have a non-traditional kind of anxiety, I call it mental looping. Most of the time, it happens at night in the dark, it’s even worse when I close my eyes. One thought will get caught in my head and no matter what I do, how hard I try, my brain will always get back to that topic. Sometimes even my dreams will loop, the exact same thing over and over again. In middle school the only thing I could think was how much I wanted everyone around me to die, but on days when I didn’t have mental looping I would be happy to hang out with anyone. In high school I couldn’t get my mind off having a relationship. The longest amount of time I could keep from thinking about it was five hours. At it’s worst, I had an imaginary friend that I pretended to date.
In college I have issues with trees. If I think of one tree being cut down, all the other images I’ve seen, things I’ve heard, every stump, fallen log, anything with wood or paper, will flash through my mind until I seep into depression. Mental looping always leads to either depression or the rage to kill people. That’s what makes it so scary.