Great guesses! I thought it was a good riddle. It makes you think about what is really special and important about us as human beings ~F
Sisyphus toils away, pushing a boulder up a hill everyday. His hard work seems forever in vain, as the rock nearing the top always falls just the same. Tired and worn, he begins once again, endlessly struggling for a means to an end. What keeps him going? How can he persist? Knowing the outcome; that there's no end to the list. It is I who frees him from his invisible tether. It is I who manifests his wanton endeavor. The struggle is timeless, the end always there, the beginning begins and whole worlds disappear. Inside, through your trials and errors foretold, it is I that will guide you and give you something to hold.
What am I? Hope is the correct answer, though many of the other answers could work as well: faith, determination, perseverance..