Your feeling guilty because you're putting her needs above your own. Don't. Assertiveness is a good thing...my reply, and your immediate thought, is standing up for your rights and what's good for you...without imposing on her rights.
You are close to the situation. That's why it's beneficial to come here and let other people help you...people who have no emotional investment in the situation.
Letting her treat you as her doormat is definately not good for you. As far as her building up resentment and anger towards you...you have no control over that...so don't bother trying. Do what you think is right, and let her think what she will.
I hope you have a Happy Easter.
"Always go straight forward, and if you meet the devil, cut him in two and go between the pieces." - William Sturgis, clipper ship captain, 1830's.