When I was a kid of barely seven or eight I think? Two recurring nightmares always managed to wake me up screaming and in tears...
One of them was from my grandparents home, which was in a village. It was a dream where I who didn't know how to climb trees, somehow found myself on one in a dark environment I can only describe as a starless night. There would always be other trees around me, with only their black, shadowy forms visible against a dark grey sky. They surrounded a pond of dark water, where I used to fall from the tree with a strangely realistic splash. I didn't know how to swim, so while I splattered and splashed trying to stay afloat, dark featureless heads used to rise all around me from the water. I woke up at that point.
Another was about me going to the bathroom alone in the night. While the previous one was in the darkness, there was an incandescent light bulb lighting up the place in this dream, and somehow that made it even scarier than the previous one. When I turned on the faucet, instead of water, a clown used to come out of it. I don't remember it's exact features, but 'it'(yes, 'it') used to smile at me, and I used to fall as my vision distorted in a weird nauseating manner. The most frightening factor was that I wanted to scream, to call out to my parents, but all I could manage was a strange sort of groan. and my sleep thankfully used to break there.
Both of these dreams were almost too realistic to be one. And in both of them, I used to be aware of the fact that they were dreams. I desperately wished for them to stop from continuing, but they didn't stop unless someone woke me up.
My point is, think about what used to scare you or does even now(sadly I am short of becoming less and less scared of things of late), and what emotions and physical reactions you experienced in your scariest moments. I try to do that. I am also more into atmospheric scares than jump scares. Yeah, a grotesque monster suddenly appearing is shocking, but I think the terror when the danger is slow but inevitable is a lot better.