No advice really. Other than your not alone. My eldest was very mature until she hit around 7. She's almost ten and it seems like she is getting younger every year. Winning, tantrums and the writing thing... Wonder if there's a connection? I think having an older friend or mentor might help but they are hard to come by. They need a good regular example of how to act. Mine won't do any of that in public but I guess is very comfortable doing it at home.
*but, not boy lol sry 4 that!
not to be rude, boy are you expecting?
if not, i'm not sure what to say, as her age seems she'd try to act older not revert....but if you were expecting, i'd say the possibility of the new baby makes her think she can act like one too....idk, i'm sorry, my stepdaughter tried this on me at walmart once trying on clothes....she was 5 though, and it only took throwing the clothes in the return basket, and walking away for her to quit...and some choice, but non-adult words to get the point across.....