As I said previously, if I would be completely strict, all I could say about reality is "I am". Not "I think" or "I am thinking", neither any other phrase. I am. Period. Of course there's no separation and no other. You are me and I am you. Our I is completely identical, with no attributes whatsoever. What differs is merely the form.
As for your question -- as kids we learn to associate "I" with personal body-mind and later in life we even identify "I" with various social (mind) constructs. Yet, "I" points at pure consciousness, thus I'm not part of uniform existence. I am that very existence itself, while also simultaneously "separate" and "unique", shaped into myriads of personal forms.
Btw. how can you be thinking or thought if there are occasions when you don't think and still are?
Pertaining to why there is something rather than nothing -- it's a moot question and a false dichotomy. Nonexistence simply cannot be. Why there are forms and manifestations of existence rather then merely unmanifested consciousness-existence is a completely different matter.
I elaborated on this in an article last year:
