It's all too familiar: we are asked to do something and we want to say "no," but because we don't want to hurt that person's feelings or make them angry, we say "yes." Then we end up feeling hurt, sad, angry, frustrated and ultimately, resentful. We have not been true to ourselves, selling ourselves a bill of goods around wanting to please someone else. In the end, we have not pleased ourselves, so we have pleased no one.
Then there is the guilt. We may feel guilty for not doing for someone what they have asked, and we may feel guilty for not taking care of ourselves. Controlling people know this--they count on it. They might try to manipulate using words to incite in us feelings of guilt about taking care of our needs first.
It's rather easy to fall prey to this cycle if we answer a request too quickly. What I find helpful is telling someone: "Let me think about that and get back to you." I sit with the request and see if it really works for me and if I have it within me to fulfill that request. Only after taking pause, can I respond with clarity and confidence: "Yes, I would be happy to do that for you." or "No, this does not suit me to do x at this time."
We are not responsible for the reactions of others. We can only be true to ourselves. Now, how powerful is THAT?!