I know the feeling. Sometimes, when people ask what I do, I simply ask them, "At what time of day?" But, I defintely took on more than my share at different jobs because I could do the work. Took years to just do what they were paying me for and demanding payment for additional chores.
I remember working for a temp agency that told me to call them the minute anything other than the request work was asked of me, so they could bill the additional charges. Wow! What a concept! That was the seed to my learning how to say 'No.'
How are you faring?