Insert boy/son and you sound just like I did a year ago. I too described my son (D) as Dr Jackyl/Mr Hyde and his moods/reactions were like a lightswitch. He reacted the exact same way when he would have a meltdown. It often took a couple hours for him to calm down once he got pissed. It was soooooo hard to deal with him. I only have one other son (C) (the boys are twins) but D would at times be mean to his brother but mostly it seems he saved the 'best' for me!! Last summer it started to get a little out of control but once D started 1st grade, it just went down hill from there. His poor teacher!
Bottom line, we had to put D in therapy because it seemed nothing we did really helped and he was just angry or inflexible about everything. After many months of therapy and a full neuopsych evaluation, it turns out he has ADHD. After long thought, we decided to try medication. OH MY GOODNESS what a difference. Like any 6 year old, he will still get angry but it's a 'normal' angry. We can calm him down, or he calms himself but in a much much shorter period of time. His grades have improved dramatically and it's just easier all around. The other day when we were getting ready for school, he comes up and asks for his meds. I asked why did he want his meds (just to see what he would say) and he replied "because they make me feel better". I was floored. He is just happier overall.
I'm not saying your daughter has ADHD or that medication is the answer to your problems, but I am suggesting that you seek help. The hubby and I really tried to handle things differently, we sought help as parents first and then finally for our son directly. I cried all too often over this and was exhausted to the bone. I am grateful for the patience of his teacher who saw the smart little guy behind the anger and never gave up on him. I'm grateful for his therapist who was always there to help him and help us!! Seek help. If you're not sure where to start, then start with your pediatrician. Ask her for a recommendation on a therapist and then go.
I wish you luck because I know how hard this is. I know that it can overwhelm you and your family and your daughters behavior can become the focus of your family. It happened in our home so quickly and since we were so overwhelmed by it, we didn't realize it until our other son asked why we were ignoring him and giving his brother all the attention. That was an eye opener. We've made a point of giving him his own time and own choices so his brothers issues doesn't overshadow how wonderful he is.
Feel free to contact me directly if you need anything else.
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julie