lisalisa, all the advice on here has been good. My son is 2 years 9 months and has had tantrums for about a year and 2 months. He's had various assessments and doesn't have a social communications disorder (eg autism, add) but has a language disorder. The speech assessor saw one of his minor tantrums and was quite worried. She also said that she can tell his vocal cords have been affected by his screaming.
Just establishing my creds here . Like you, I'd tried all the usual suspects for dealing with tantrums. What's ended up helping is something my SIL suggested and the speech assessor refined. It's basically this: praise them like mad. My SIL said to praise him for things that he's about to do, even if it's just saying (as he's opening a door) "Son, could you open the door please" then immediately follow that with lavish praise for opening the door. That has helped his general temperment a bit and mine a lot. The speech assessor said that instead of completely ignoring a tantrum, I should tell him that it's unacceptable to be screaming and then ignore it. After that, at the very first opportunity (ie long deep breath), start praising him for stopping the tantrum. She did say it's a case of "meticulous timing"; I don't want to reinforce the tantrums. It's been working, though! I praise him for not tantrumming, too.
Instead of putting her in a room, could you put yourself and your daughter in a room and do something fun? Prepare ahead. I haven't tried that technique, my son is a scream, lie down and beat the floor type guy, which is easier.
I've got an older son, too (he's 6 1/2) and he realizes that his little brother has ishoos, so is willing to give him a bit of leeway with food, especially. So my youngest doesn't eat the same things as the rest of us.
Hope this doesn't sound all cool, calm and collected. Last November, my sweet husband took my youngest to his parents (from Switzerland to the US) for 2 weeks to give me a break. It was an emergency evacuation. I'm on ADs and my GP has taken over my youngest's case (in terms of finding therapy, more tests, etc) as it's the root cause of the depression. Pathetic, I know, people have a lot more to deal with and handle it better, but... there you go!
Thinking of you.