This is a very strange rec and might be too weird/not horror related enough but, Altered States is a very interesting movie that takes a weirder more stylistic approach to psychological horror. I would also rec some David Cronenberg movies, here’s a few that I really like: The Fly (1986) Videodrome Rabid Shivers - warning a bit of a rough watch Slither - not a Cronenberg movie but very Cronenberg inspired (it’s a more fun watch if you’ve already seen slither since it visually references it)
Black Phone & Smile have been two recent hidden gems for me. Loved both. Still need to see Smile 2 though Not necessarily a horror movie in the traditional sense at all but Shutter Island is a psychological thriller and it’s fantastic. The Mist is another older hidden gem that’s gut punching
Smile 1 was so great. I didn’t like Smile 2 as much, but it was still a good watch. Psychological thrillers are also up my alley, so I’m not mad at recs for that. Unfortunately, fell asleep during Shutter Island and that ruined the movie for me haha. Ooo The Mist! That’s going towards the top of the list.
I would also have to rec the first two Hellraiser movies and Nightbreed by Clive Barker and I’ll also through out a more artsy/film nerdy horror genre that you should try checking out, the Giallo genre (Italian slasher movies), some good introductions to this genre are some of Dario Argento’s films like Suspiria or Tenebrae