Which dissociative disorder involves a person feeling detached from his/her mental processes or physical body?

Depersonalization

Explanation:

Depersonalization is the phenomenon in which the person feels detached from mental process or from one's body.

It basically deals with the feeling of observing the nature from outside. The feeling of being detached from everyone and seeing the nature from one's point of view.

This disorder is often cured by drugs and psychotherapy due to depression. This is a type of depression in which the person creates its own imaginary world and starts feeling detached.