How can I deal with it hurts to talk about it?

I'm not sure how old you are, but you sound quite young. It must be horrible to hear someone else calling your dad "dad", but you have to understand that she sees him as a father and isn't calling him "dad" to annoy you. As horrible as it makes you feel, you are going to just have to deal with it. You didn't say if your mum was still around anymore... are you close to your stepmom? Do you call her Amy or mom? Don't be sad :-) Be happy that your dad is a good dad and someone else feels that they can call on him. Be proud that he's your dad, don't see it as taking sides, he has to be fair.