You may have a dissociative disorder, probably depersonalization. I have dissociative amnesia (not remember blocks of my life, I don't remember much that happened in the last 15 years, give or take a few random events) and a form of depersonalization where I don't recognise myself in the mirror, but nothing too severe. I do sometimes feel like there is a part of me that's missing, a part of my mind that everyone else has that makes things feel "right".
If I get very "withdrawn" from myself I consiously think something out, with intense effort, like how exactly I would re-arrange my bookshelf or how I should try go about studying next year in school, the fact that I feel like I'm forming these thoughs myself helps ground me to reality. Your symptoms might also be caused by a delusional disorder, from the schizophrenic spectrum (schitzophrenic, affective, typy, so on...), but I'm not a doc. If it's very pathologic I'd get it checked out, but tbh I need to go to the doc myself but I'm too moral to tell my parents there son has every mental illness in the book (but I'm still suprisingly High Functioning), so don't take my advice
good luck
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