I just wanted to chime in here. I have a daughter who is very much the same way. She will be 5 in September. If we start with something that is a minor offense and I bring her to her room for discipline, sometimes she gets so upset it goes from what could have been a quick discipline session to a 20 (or up to 40) minute event. She is VERY strong-willed. She thinks she's a grown up (she's actually said as much to me)! She still hasn't completely accepted that I, as the adult, get to control things more than she does.
That being said, do I have answers? Not really. I read some of the other posts and was able to find some good advice, but not much I haven't heard or thought of before. I will say that I've seen improvement over the years. Tantrums, in general, are less frequent than when she was younger and the big long ones are even less frequent. The only thing I've found is that she needs an appropriate balance of two things: 1) very clear and consistent discipline (which is sooooo hard when you have a child like this) and 2) lots of love and reassurance, especially after being disciplined. I've also found it very helpful to give clear instructions on expected behavior--specific actions and words that ARE appropriate in the place of whatever she did wrong.
The one thing I've had to really get through my own thick skull is that, despite the fact that she is extremely bright and picks things up very easily, it's not the same with learning appropriate behavior and emotion control. It takes a very long time and lots of practice and repetition for kids to get this.
Hang in there and good luck,