@chibi ren,
So she isn't being nasty, she isn't putting you down, she isn't condescending, she isn't actually doing anything to damage your self esteem.
Self esteem is worked for, rather than a given. It's the worth you place on : your character, your beliefs, your emotions, your sense of humanity...but it's never truly been about comparison...it's about what you value, and what you see of that in yourself.
The greatest way to build self esteem is to identify what you value (in being a human being), and go about building that into yourself.
It will, of necessity, include standing up for yourself (respectfully).