This thread is really helpful for me too. DD (21mo) is very very verbal and speaking in complete long sentences but will STILL get fixated, just like you were describing, on one thing and just not let it go.
This AM she threw a fit for 1 1/2 hours (screaming/kicking/demanding/trying to bargain because she wanted to take her pacifier outside of her crib (we do not let her but DH forgot and let her bring it into our bed this AM... I had to then go back and try to get her to put them back in the crib). I am so tempted to just throw them all away because they cause so many issues but DD just potty trained a month ago and I am worried that if I take them away now it will be too much change in a short amount of time.
She will throw full-fledge fits like this (usually not lasting as long, usually 15-30min or so) at least once a week. Whether it is not wanting to get in her carseat, wanting a fruit snack, ect. She has very high-pitched screaming, and always has from the day she came home from hospital. She has also ALWAYS been just as demanding/intense as she is now. If once, just once, there is an exception (like today when DH let her take the pacis out of her crib) she will loose it if we try to go back on it then test it out for at least a week after. She doesn't forget! I am dreading this week and the paci issue... Poor DH didn't even know what went wrong!