W..
8 is no longer considered precocious puberty. isn't that scary? Typically the hypothalamus revves up about 18 months before a girl gets her period. LOTS of girls start their period at 10, so she's not out of line to be getting "hormonal" at this age.
I started managing my daughter at 8 like I managed her when she was 3. Separate MY emotions from HER behavior. Give her simple commands. Ignore tantrums and reward positive behavior. Teach her that how she FEELS and how she BEHAVES are 2 completely separate things and give her space to feel whatever she feels, but coach her on how to ACT when she is around others (again - stress that feelings and emotions are 2 different things). Let her have as much control over things that are appropriate, so that she WON'T be successful gaining control over things that are NOT appropriate.
I call 3 year olds tiny terrorists in a lot of posts I answer. I think pre-pubescent girls are just the life-size version.
You have a small window of reprieve when she starts her period - because you aren't the enemy when she needs you.
But then it's going to ramp up again. When the sex hormones kick in. Same advice for that as above.
Are her brothers older or younger?
Good Luck
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ETA - I read a book that talked about how their bodies in puberty are getting used to the release of endocrines, and their brain doesn't always get it 'right'.... so their responses aren't appropriate. ie- something that SHOULD make them mad doesn't because their brain didn't release enough of that hormone. But something that SHOULDN'T make her giddy does... because at that moment her brain released TOO MUCH of that hormone. Sometimes it helps if you can explain to her what is going on so she knows she has to be more diligent about separating how she feels she should respond from what is appropriate action.