Oh Why, Oh Why, Did my bug Turn Five?

If anyone tries telling you parenting is easy, they aren't one. This past few days around here has been HARD. Bug is going through a stage of being downright defiant. It is taxing DH and I to near breaking...I cry out to God for help, I honestly wonder if he hears me.
If there is one thing I want my child to learn more than anything else its to be obedient. In her future, there will likely be a husband, maybe a boss, we will be here for for several more years, and many other authority figures surround us.
She is learning of consequences. On one hand I hate seeing my child upset, but for reasons beyond my comprehension, when things do not work, she loses her temper and hits or just outright tries to hurt us. I am nervous, at times, when we think about the fact we are trying to have another baby (have been for 5 years now) because she has these horrible mood swings into violence.
We are firm, non yielding parents. Her words cut like a knife "I was being good, yet you were mean to me" generally the response we get when we say no to a request, be it reasonable or not. Most of the time my response is trite, that you have to be good more than just when you want things, and I put away what she is begging for....guess what that gives us....can you guess?? exactly, MORE FITS. I have tried all sorts of discipline, nothing seems to give permanent results. most of the time now, I just ignore the fits and make her take some alone time. Hopefully she will learn something from all of this, if not, I seriously pray that this is just a phase and "THIS TOO SHALL PASS"
Oh why, oh Why, did my bug turn five?