Certainty of uncertainty

How can we be certain of anything? This is Descartes main focus on his Second Meditation. Both Descartes, and The Matrix, challenge our beliefs of reality, and how we perceive it. Arguing that we consider reality  the information we process with our senses, we base reality on sensory information, yet reality can't be that limited. One of the most memorable scenes in the Matrix is the first conversation between Neo and Morpheous after Neo is freed from the Matrix, where he challenges Neo to tell him what is real, arguing that if reality is sensory information, then it is only electrical signals interpreted by our brains. For a moment, let's pretend reality is solely based on our sensory information, then the lost of one of our five senses would imply we lose our ability to experience reality completely.
How can we be certain of anything outside of our "thinking," consider this, before going to sleep, we cannot consciously say or remember, the last moment before falling asleep, we only have conscious when we wake up, in the same way I do not have conscious of the moment I was born, the moment I first appeared, or the moment I acquired this body. I have memories that can go as far back as when I was 4-5 year old, but I cannot experience being consciously aware at that moment, this is why we cannot be certain of anything outside our "thinking," because same as imagining the future, memories can be only the product of my imagination.
I exist because I think, because I doubt I exist, I am existing, or as Decartes says "So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." 
The only certainty for Descartes is that "I" exist, what is this "I" is another question to which an answer is missing