"Life is but a dream within a dream."
Ah good o' Poe.
Dreams and the questions of self and reality or existents are always fun.
So let's start with this. How do you define reality for yourself? Is it what you can experience through your senses? Or is it something more than that? Do you define it by your bouncing around other humans and things and getting response and reactions out of them? Do their thoughts of you help define that yes you are in fact here. An in so doing say to yourself that this is reality and you are existing within it.
Let's say we toss your dreams into this mix now. Is the dreaming you real? You have those dreams where you can touch and think and all of that. So is that version of you real as well in someway? Is it a echo of something that could be or even is in another place and time?
So do we go with that old saying you have quoted up there of "I think, therefore I am" or do we need something else? Because here is the fun idea, can you prove that you are not just another random collection of thoughts and ideas that is floating through someone else's mind.. A voice that is just another echoing ghost in that great expanse of mind.
But that's just my rambling in the dead of night.