One may call the set of his/her habits and beliefs a Self (or ego, or "who I am"). This is a convenient convention; habits and beliefs (aka "thought habits") tend to persist for some time and that matches nicely the sense of Self. Moreover, what one usually does and what one believes in would probably be the natural answer when asked to introduce oneself.
However, habits can be strengthened, weakened, adjusted. Beliefs may (and do) change too, through time. Old habits and beliefs may be dropped; new ones may emerge. One's Self has no more solidity than such loose set of malleable habits and beliefs.
There is no ego "resisting" or "fighting back" when one works on letting go, on un-grasping, on giving, on being fearless... It very much feels like there is such a Self resisting, but that's a story we make (a conceptual theory based on feelings). The Self is a collection of habits and these habits are just that, habits i.e. well-worn, least-energy, paths. One can change habits one by one (that's so much more manageable than attempting to change the complete set of them in one go), so one can change his/her Self.