what I usually do when I'm trying to sleep and I'm imagining something scary and/or disturbing, I imagine myself beating the crap out of whatever it is, then it usually goes away, then If it comes back, I rinse and repeat.
try this, and fight as badass as you can, it usually works better.
if you can't, your one of two things
1. lack imagination
2. don't deserve the grace of the magical lava fish.
3. both