I can understand where you are coming from. Both of my boys are prone to throw fits, but they used to be quite bad. My oldest son used to go as far as to curse, and hit me. It has taken us almost 4 years to break this habbit. And it has taken alot of time, and a LOT of patience. And I can understand how it makes you feel as if maybe you are failing. But I think kids tend to push boundries to see just how far they can get, you just have to be consistant. There are alot of different approaches you can take, but I will tell you what we did, and maybe it will help.
First off, we had a "family meeting". We sat down as a family, and discussed the things that where going on. Its hard with younger children, but they tend to find a way to say what they want. We set down a set of rules, like "no hitting","no yelling" and such. And we also set down a list of consequences. You just have to make sure to stick to it. The way we set ours up was, lets say they fought, first offense was a verbal repremand "you know you arent supposed ot do that, etc." seocnd offense was time out (1 minute for every year they where old). Offenses after that where things like no tv for the rest of the day, and so on. Bascily we found taking things like their favorite toy or show away worked better than anything. When it comes to time out though, we found it best to make them sit on their bed for time out. If we let them stuck their nose in a corner like alot of parents do they would watch tv or play with a toy.
And it did take alot of patience, and some headache medicine too. Because inevitably, if we got to time out, at least for the first while, we had to put them in "time out" and of course they yelled and so on. And it was hard, because they yelled all sorts of the typical things kids do when they are mad, "you're mean", "i dont like you" etc. But after a while, it finally got to where most of the troublesome behavior was gone, because they knew there where consequences to their behavior.
I dont know if this will help you, but I hope it does.