how did she become the boss?

How is that I, the adult in this situation am completely cowed and bullied and bossed by her the two year old who whines in the most mind numbingly painful tone, cries so loud I'm sure the neighbors thing I'm whipping her with a garden hose and repeatedly covers my mouth while I'm talking and tells me to "stop talking?" I used to be able to take a firmer stance and come out the victor, now I roll over like a drunk passed out on the curb at closing time.