I have used Atlas to refer to myself. I picked up the usage from a Spider Robinson short story (not a Callahan's story). The meaning is a figurative analogy - one who has the world on their shoulders. A person with the world on their shoulders wants to change the world, to make a difference -- and feels extremely guilty about every shortcoming and self-perceived failure within them.
Being an Atlas in this sense tends to mean that you're overwrought and overstressed about things that you have no control over and things you cannot change (the past is a good example of one of these). There is one thing an Atlas must do if they want to accomplish things -- and that is let it go.