You can't directly replace "I'm obsessed with horror movies" with "My mind is watching horror movies," as the latter doesn't convey the same meaning. "I'm obsessed with" suggests a strong passion or fixation, while "My mind is watching" implies that you are thinking about or imagining horror movies but doesn’t communicate the same intensity of interest.
If you want a similar expression to convey obsession, you might say:
* "I can't stop thinking about horror movies."
* "I constantly have horror movies on my mind."
* "My thoughts are consumed by horror movies."
Let me know if you need more options!