In my early 20's, I became convinced that I was being haunted.
Weird shit would happen all the time.
I worked nights, so I'd be driving home around 2AM and as a streetlight slid across my backseat maybe I'd glance in the rear-view mirror and see a human figure in the back seat. Turn and look, and nothing there. Check the mirror, still nothing. 5 minutes down the road, I glance in the mirror, and there's the figure again. Didn't happen every day, but happened at least once a week for several years. It terrified me.
I also started seeing human silhouettes in the trees in front of and behind the house I grew up in. I wasn't the first person to notice this, actually, an ex saw it before I did, but after that for years I wouldn't even look out the windows at night lest I see a human shadow among the branches.
One particular occasion, I have the figure show up in my back seat a few times, and when I park in front of my house, I can feel this... force. This heavy, energetic, angry, malevolence, surrounding the car. I couldn't make myself get out of the car. I was convinced I would be harmed if I tried to leave the car, and had to call a friend and talk to him while I got out and got to my house, and when I got inside I turned on all my lights and pushed shit in front of the doors.
Later, after moving out, I'd hear whispering in the walls. Initially I had thought it was my neighbors and they hated me, because the whispers were always super hateful toward me, critical and kind of aggressive. Then one day I heard the whispers talking about something it would be impossible for anyone but me and my then-wife to know about. When I realized nobody could actually know what I was hearing, it scared the shit out of me.
It was later that year I learned that hallucinations are one of the less common symptoms of bipolar disorder. Been medicated for about 3 years now and haven't had any spooky shit since then.