I never talked about this to anyone. My T knows a little...very little. It started when I was 16. At the time I was very isolated, no friends, and super anxious because of school and socializing, I never spoke to my classmates and they laughed at me or treated me like a child. I experienced this sensation of not feeling the world real. Like, I remember for example, I was in class, it was like my surroundings were fake, the things around me, their colors, seemed strange, like they didn't have a reason to be there. Everyone was slow, I walked and nothing made sense (people and things around me).I had some "episodes" of this, they only lasted some seconds and then went away. Then I read it's called derealization when you feel that external reality isn't real, something isn't right, and depersonalization, when you feel you and your body are not real. I had more derealization than depersonalization I think.
The last episode I remember happened 2 years ago, I was walking and felt like my surroundings were fake, like from a film. Then after some seconds it went away.
In my situation, I alway experienced this when I was very stressed out and depressed. When I feel good I never have this sensation.