Well, it IS you. It is a part of you. Not a foreign body or alien life form or anything like that. (I know you're not really thinking that). It is just a part of you that you have lost contact with... not been aware of. Not too much different to a forgotten memory.
I know how freaky and scary it is. It will be okay. You will be okay.

I think it can be very overwhelming at first to acknowledge there is stuff there that you don't know about, or don't have control over. There are so many implications of that and none of them are very reassuring! But take deep breaths, go for a run, do the things that normally calm you and help you find your feet. It really will all be okay in the end. You can do this.