Boy, can I relate. My four-year-old boy is an extremely intense, persistent, sensitive individual, and we've been dealing with extreme tantrums, etc., since he was only a year or so old. He's the quintessential "spirited child." One book that has helped me gain some insight into his behavior is "Raising Your Spirited Child" by Mary Sheedy Kurcinka. Spirited children are just more challenging than other kids, but of course, loving them can also be very rewarding as well -- especially if we can help them learn to manage their feelings and behavior. I read another book recently about "strong-willed children" that spoke about their need to understand the "why" of a situation more than other kids. "WHY can't I climb on that?" "Honey, you might fall off" -- instead of just "Get down from there now!", etc. So I often try to give my son some brief explanation, within reason, to garner his cooperation.
Our little boy, like your daughter, can change from a sweet, endearing little angel into a raging, shrieking, kicking, out-of-control tyrant the next. The tantrums can last a loooong time, at high intensity, and he often seems to have no capacity to regain control, no matter what extreme measures we are forced to take (short of hitting him, which we don't believe in.) His behavior also seems to go in cycles; he'll be cruising along and pretty easy for a while, and then *wham* -- totally out of control for maybe a couple of weeks. For a long time, we thought it was a typical childhood phase that would pass. Recently, we've come to believe our boy may need more guidance with his behavior than we've been previously able to give him. We heard about a program called "The Total Transformation" (http://www.thetotaltransformation.com), and when we listened to the testimonials, something struck a chord. We just ordered it, but have not yet received it, so I can't give you any personal feedback as yet. They have a 30-day money-back guarantee, though, so we figured we'd check it out. We'd rather be proactive and try this, instead of just hoping our son's extreme displays of temper will go away on their own.
Some other observations are that if we as parents are tense or angry about something, our little ones act as mirrors and reflect back those emotions. Also, on the days we have more structure, things seem to go more smoothly. My husband and I tend to be more go-with-the-flow types of people, so this is sometimes a challenge for us. But it helps kids immensely if they know what to expect. We also try to employ a lot of the "Love and Logic" techniques -- giving them some control within your chosen limits, using "enforceable statements," natural and logical consequences, etc. Although quite useful, it hasn't been quite enough to completely avert the mega-tantrums in our case, however, which is why we're checking out the program I mentioned. Like you, we've tried plenty of consequences, removing privileges, etc. And incentives, too -- I sometimes get little $1 (and some bigger) things, or plan special outings or activities with me or my husband, that I know my son will like and then he has something to work toward with his behavior. (I don't want to constantly be "bribing" him, though!)
Your toilet anecdote hit home -- we once took our newly potty-trained son to a function and my husband flushed the toilet without thinking and our son had a 20-minute shrieking fit that he didn't get to flush it himself. EVERYONE at the fuction heard the ruckus. Luckily, many were parents themselves and quite understanding, but it didn't help that it was during a eulogy at the memorial service for a friend's mother!
Well, obviously your topic has hit home with me, because I seem to have written a book. I hope even a little of this has been helpful -- at the very least so you know you're not alone.
Parenting is such a challenging journey! But I think we're here to learn just as much as they are. Good luck to you!
K.