Jess,
You are not a liar, however many abuse victims feel shame because they feel they could have prevented the crime against themselves. When an adult harms a child it is always the adults fault. I do bleieve that the wonderful members of this forum want you to heal from this horrific ordeal. Many of us are parents so if you are right about being a liar that makes all of us wrong. And sweetie I don't think all of us could be so far off base.
You need a therapist because she is an expert in her field. When you become sick, you also see a doctor who identifies your illness and prescribes you pills. The medication does your healing. Healing from intrafamilial sexual abuse is different in two important aspects. The right "pills" are in your mind, and your mind is not working right. This is why you need a therapist. She will lead you through your treatment, explain things to you, and help you resolve your traumatic experience. She is not there to judge. She is there to help you know matter what your feeling.
It sounds like your Dad is very concerned about you so please talk to the therapist like you do to us. You may think you are able to fool your Dad and stepmom about how your feeling but I suspect they have picked up on telltale signs. I have maintained a happy face for so long that I can recognize when someone else is doing the same thing.
Sending you many gentle hugs,
Kitt