And I'm glad you brought this old thread back to life, so to speak, Hatoun_alth!
As I said in my old post, low serotonin levels coupled with a stimulating subject can be a cause for rumination and insomnia.
How do we get out of that cycle?
We are free to choose the thoughts on which we dwell. I equate thoughts like books on a shelf- I'm free to choose what book I wish to read.
But if we are focused on a thought, anything that breaks the cycle of rumination will do the trick- From praying, meditation, or just thinking happy thoughts about pleasant experiences.
Here's a technique which works unfailingly for me:
First, I close my eyes and focus on my breathing, inhaling to a count of a count of 8 to 12 and exhaling to the same. Once the rhythm of my breathing is established, I focus on the flashes of light I see when I close my eyes. Those flashes of light are called phosphenes caused by random firings of the optic nerve. Like seeing animals in cloud shapes, I attempt to "see" things in the phosphenes.
Phosphenes are so cool- they're like a moving swirling picture collage. If I focus on my breathing and phosphenes, I invariably fall to sleep.
However, thinking about death doesn't freak me out. What freaks me out is the dread of the pain of living!
The best to you, Hatoun_alth!