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My bet is that she's been struggling with this. Maybe she wasn't ready to articulate, "I love the BF, but wish his daughter would disappear off the face of the planet." Lots of people have trouble recognizing unpleasant truths.
So, LW wrote to Cary. In the course, she's probably figured out that she doesn't like Anna. Good. Now she knows. It's a hell of a lot better that she wrote than if she hid her feelings from judgmental Saloners AND HERSELF!
It's no crime to dislike your partner's kid. It's common. An old African saying about living with step-children is, "Other people's children are like other people's nasal mucus." Let's not forget all the fairy tales with wicked stepmoms.
Now that LW has acknowledged her feelings she can make some good decisions about what to do about it, as per Cary's advice.