frustrated, resentful, indignant

where do i draw the line between serving someone as Christ calls us and being a doormat, only to be used and walked over, discarded when no longer useful? when i am not seen as a human but rather as a means of transportation, why do i feel so torn to say "no"? does this feeling mean i'm supposed to say "yes" every time someone wants a favor from me? ...even if, in my eyes, this person is just trying to live according to a selfish way of life and i am just promoting it by giving in, basically saying "your wish is my command"? should i just go along with the request or go against it? why do i feel shitty either way i respond to the situation?