Bad Behavior, Tantrums and whining, Oh My!

Well, the last couple of weeks have been tough. Little Bee has developed split personalities! In one minute she is the sweetest, kindest, most loving little girl there is. She plays and strokes my hair, gives me hugs, tells me she loves me, all the little things that sugary sweet little girls do. The next minute, oh my, a little monster is standing in front of me telling me no, sassing while pointing her finger at me and hitting me! And a few times when I have asked her if she wanted to go to the naughty seat she said, "yes!!" Yesterday (and let me preface this by saying that a friend of mine, who has a little girl just a few weeks older than Bee, said her daughter did this and I thought in my head that there is no way MY child would ever do this, well boy was I wrong) little Bee was being a little monster and not picking up her toys. I told her she was going to go to the naughty seat, so what does she do? She said "okay" and walked over and sat herself in the naughty seat!!! I thought I was going to die! Everyone keeps saying "oh, she's only 2, she doesn't know what she's saying or doing" but I am sorry folks, that child is a lot smarter than you give her credit for! All of the parenting advice I read says to stay calm, don't react to it, don't engage. Don't engage? Don't engage while the child has thrown themselves on the ground screaming "No, mommy" while all of the windows are open! Or is screaming through the grocery store because I won't let her smash the bread! Yeah right! How am I supposed to remain sane and calm through all of that? And don't even get me started on the whining! She can not ask for anything with out whining! I do as all the reading says and say, "I can't understand whining, you need to use your big girl words" or "You need to ask mommy nicely and say please." When I try to talk to people about it I hear, yet again, "she's just a baby" or "she is still so young give her a break" Give ME a break! Sorry, but learning manners has no age limitation!!!! I just want this period to pass and to have back my sweet little helper who did anything I asked of her! Where did that child go???? I feel as though, since I have been venting, that I need to say that I love my child very much and she is frankly the reason for everything I do now! But, really, the two's is tough!! And please don't say, "You'll see, the three's are worse and so are the four's," blah, blah, blah, let me wallow in my own whining!!