Learn to say no withOUT saying “sorry,” first. You do not need to apologize for NOT being someone’s shlepper 24/7. You can say, “I wish I could, but I am not available,” or, “I have other commitments that I must take care of, but if I have some free time, I will do my best.”
The fact that someone expects you to to do something for her, does not obligate you to do so. And this advice is coming from someone who was exactly in your position, and did exactly the same thing as you. I finally started to feel VERY resentful of the time these unending “little errands” were taking away from my family (and my own personal free time), and came to realize that if you cannot do something with a whole heart, perhaps it is time to reassess the situation and step back. Especially so, if the woman has a husband who could and SHOULD be doing these things for her.
Doing chessed is awesome, but you need to do chessed for your husband and children also, and it sounds from your description, that your friend is interfering with that, albeit possibly unknowingly.