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So how can you be certain you have free will?

Quite sure...

If I did not, I would probably never had this hunger for knowledge because most people do not.I believe that, at least, is a choice I made myself. I rather believe that life is endlessly complex and the interconnectivity is what causes the sense of losing control. 7 billion individuals mean that at least some will influence you, but the level of influence is your choice.

your statement negates itself.