I haven't been as close with my stepdaughter & I feel terrible.

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Is it normal for me to feel distant from my 3 yo step daughter? I'm 37 weeks along and during my pregnancy I've just been really irritated.. I love her to death, but I don't know why I feel a little resentful towards her....? I know she's an innocent little girl, and I know this is all me. I'm wondering if anyone else has been through something similar... Sometimes I fear that my daughter won't be loved by her dad as much as my step daughter. Has anyone experienced something like this?