Devil in disguise

In the last week, my beautiful little angel child has become possessed and has turned into a devil child. We are about 2 months away from 2 and she has jumped onto the fast track to the terrible twos. Any little thing will set her off into a full on flip-out, throw myself on the floor, screaming, crying, I don't care where we are, I don't care who is watching, lasting for 20 minutes, melt downs. She does this at home, in the car, at the office, in a store, at a restaurant. Yesterday when we got to the office she cried for 20 minutes because she wanted a book that was in the car. Never mind the box of books, toys and videos she already has in the office. It is unbelievable frustrating! I can't figure out the best way to deal with her temper tantrums. I have a feeling that she is getting smarter and she is testing me and PR Dad to see how far she has to go to get what she wants. I can hold out and let her cry for 20 minutes and not give in. PR Dad is a completely different story. And it is laying the foundation for a household just like the one I grew up in. Where Mom was the mean and nasty one who always said No. (No worries Mom I totally understand now!) And Dad was the "Yes Man". Which means that PR Dad will always deal with a happy spoiled child, while I will have to deal with a bratty spoiled brat. We have talked about this before and we don't want The Phi growing up a spoiled kid, who gets everything she wants or who throws a fit when they don't. What is the best way to do this??? Help me out people. I want my little angel back!