Can you be sure that you yourself exists?

You seem confused. You intertwine existence and free will as if they are the same thing. I know I exist. As you pointed out, "I think, therefore I am". Free will is a separate question. Your argument there seems to be implying that our free will is merely an illusion created by cause and effect. I agree. It doesn't matter to me though, as no-one is capable of processing the infinite amount of data required to predict our thoughts and actions. Therefore the illusion of free will is to all intents actual free will.