Is it normal for me to feel resentful of my MIL?

I am the one primarily responsible for the care of my mother in law. My husband (her only child) does not have a good relationship with her. She has never liked me and at one time wanted him to choose between the two of us. Fortunately for me he stood up for me. She is a very emotionally driven person and often takes out frustrations on me. I do what I do for her because I love my husband and I do pity her. Her abrasive personality has kept her from having longtime friends. Ironically, she truly believes she is a sweet little old lady and everyone thinks she is cute. People that spend much time around her realize the shallowness of her personality and admire me for my efforts. I don't feel admirable, I feel guilty that I dislike her so much. I just pity her, and she depends on me for everything. I am kind to her and treat her with respect, but once I leave her apartment (she lives in a retirement home) I find myself thinking uncharitable things. Is what I feel normal? I don't wish her dead, but I do look forward to the time she is not part of our lives.