What you describe sounds like a form of dissociation. Everyone experiences it to some extent, like daydreaming or "spacing out," but there are different forms of it varying in severity. One is depersonalization, which is when a person feels disconnected from themselves, or like their body isn't theirs. Dissociation is often a response to a traumatic experience, or when a person can't emotionally handle something... I think... I'm not positive I'm explaining this very well or completely accurately, and I just read about it yesterday so further research would be beneficial if you want to know more. :p
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Diagnosis: Major Depressive Affective Disorder, Generalized Anxiety Disorder, Mild PTSD, Agoraphobia with Panic Disorder, Attention Deficit Disorder of childhood with hyperactivity (more inattentive, though), Mild OCD, Social Phobia, Tourette's Syndrome