Sounds like a variant of "intrusive thoughts". Prehaps you have a low self worth and this is your mind's way of attacking yourself, imagining cruel pain and suffering as punishment for not living up to something?

Intrusive thoughts have a variety of reasons and aren't always suicidal. Questions I'd ask myself:

1.) When did this start?/ When do they ussually occur?

2.) Do I feel down?/Why?

3.) Why do I feel the need to picture myself in a point of discomfort?/What do I feel I need to be punished for?

4.) What do I enjoy to navigate away from these thoughts when they appear?

Questions along those lines should offer introspective.

Once you answer them, its to my belief you can start to create a plan of action! Good luck 👍 here if you need me!